content: <PAGE>   1
                As filed with the Securities and Exchange Commission
                                   on June 5, 1996
  
  ================================================================================
  
                         SECURITIES AND EXCHANGE COMMISSION
                               Washington, D.C. 20549
  
                                      FORM S-8
  
                            REGISTRATION STATEMENT UNDER
                             THE SECURITIES ACT OF 1933
  
                                    SEPRACOR INC.
               (Exact name of registrant as specified in its charter)
  
             DELAWARE                                           22-2536587
  (State or other jurisdiction of                            (I.R.S. Employer
   incorporation or organization)                           Identification No.)
  
                  111 LOCKE DRIVE, MARLBOROUGH, MASSACHUSETTS   01752
                 (Address of Principal Executive Offices)    (Zip Code)
  
                           1991 DIRECTOR STOCK OPTION PLAN
                              (Full title of the plan)
  
                                TIMOTHY J. BARBERICH
                                    SEPRACOR INC.
                                   111 LOCKE DRIVE
                          MARLBOROUGH, MASSACHUSETTS 01752
                       (Name and address of agent for service)
  
                                   (508) 481-6700
            (Telephone number, including area code, of agent for service)
  
                           CALCULATION OF REGISTRATION FEE
  
  <TABLE>
  <CAPTION>
                              Proposed    Proposed
  Title of                    maximum     maximum
  securities     Amount       offering    aggregate       Amount of
  to be          to be        price       offering        registration
  registered     registered   per share   price           fee
  - ----------     ----------   ---------   ------------    ------------
  <S>            <C>          <C>         <C>               <C>
                                                            
  Common Stock,   75,000      $14.44(1)   $1,083,000(1)     $374.00
  $.10 par        shares
  value
  </TABLE>
  
  - --------------------------------------------------------------------------------
  
  (1)   Estimated solely for the purpose of calculating the registration fee, and
        based upon the average of the high and low prices of the Common Stock on
        the Nasdaq National Market on May 30, 1996 in accordance with Rules 457(c)
        and 457(h) of the Securities Act of 1933.
  
  ================================================================================
  
  <PAGE>   2
  Statement of Incorporation by Reference
  
          Except as otherwise set forth below, this Registration Statement on 
  Form S-8 incorporates by reference the contents of the Registration Statement 
  on Form S-8, File No. 33-44808, relating to the Registrant's 1991 Director 
  Stock Option Plan (the "Initial Registration Statement").
  
  PART II. INFORMATION REQUIRED IN THE REGISTRATION STATEMENT
  
          Item 3 of the Initial Registration Statement is amended and restated
  in its entirety as follows:
  
          Item 3.   Incorporation of Certain Documents by Reference.
                    -----------------------------------------------
  
          The Registrant is subject to the informational and reporting
  requirements of Sections 13(a), 14, and 15(d) of the Securities Exchange Act of
  1934 (as amended, the "Exchange Act"), and in accordance therewith files
  reports, proxy statements and other information with the Securities and
  Exchange Commission.  The following documents, which are filed with the
  Securities and Exchange Commission, are incorporated in this Prospectus by
  reference.
  
                  (1) The Registrant's latest annual report filed pursuant to
          Section 13(a) or 15(d) of the Exchange Act, or the latest prospectus
          filed pursuant to Rule 424(b) under the Securities Act of 1933 (as
          amended, the "Securities Act"), that contains audited financial
          statements for the Registrant's latest fiscal year for which such
          statements have been filed.
  
                  (2) All other reports filed pursuant to Section 13(a) or 15(d)
          of the Exchange Act since the end of the fiscal year covered by the
          document referred to in (1) above.
  
                  (3) The description of the Common Stock, par value $.10 per
          share ("Common Stock"), contained in a registration statement filed
          under the Exchange Act, including any amendment or report filed for the
          purpose of updating such description.
  
          All documents subsequently filed by the Registrant pursuant to Sections
  13(a), 13(c), 14, and 15(d) of the Exchange Act prior to the filing of a
  post-effective amendment which indicates that all shares of Common Stock 
  offered hereby have been sold or which deregisters all shares of Common Stock 
  then remaining unsold, shall be deemed to be incorporated by reference herein
  and to be part hereof from the date of the filing of such documents.
  
          Item 9 of the Initial Registration Statement is amended and restated in
  its entirety as follows:
  
          Item 9.  Undertakings.
                   ------------
  
          1.   The Company hereby undertakes.
  
               (a) To file, during any period in which offers or sales are being
          made, a post-effective amendment to this registration statement:
  
               (i) To include any prospectus required by Section 10(a)(3) of the
          Securities Act;
  
               (ii) To reflect in the prospectus any facts or events arising
          after the effective date of the registration statement (or the most
          recent post-effective amendment thereof) which, individually or in the
          aggregate, represent a fundamental change in the information set forth
          in the registration statement; and
  
               (iii) To include any material information with respect to the plan
          of distribution not previously disclosed in the registration statement
          or any material change to such information in the registration
          statement;
  
          PROVIDED, HOWEVER that paragraphs (i) and (ii) do not apply if the
          registration statement is on Form S-3 or Form S-8, and the information
          required to be included in a post-effective amendment by those
          paragraphs is contained in periodic reports filed by the Company
          pursuant to Section 13 or Section 15(d) of the Exchange Act that are
          incorporated by reference in the registration statement.
  
                  (b) That, for the purpose of determining any liability under
          the Securities Act, each such post-effective amendment shall be deemed
          to be a new registration statement relating to the securities offered
          therein, and the offering of such securities at that time shall be
          deemed to be the initial bona fide offering thereof.
  
                  (c) To remove from registration by means of a post-effective
          amendment any of the securities being registered which remain unsold at
          the termination of the offering.
  
          2.      The Company hereby undertakes that, for purposes of determining
  any liability under the Securities Act, each filing of the Company's annual
  report pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and,
  where applicable, each filing of an employee benefit plan's annual report
  pursuant to Section 15(d) of the Exchange Act) that is incorporated by
  reference in the registration statement shall be deemed to be a new
  registration statement relating to the securities offered therein, and the
  offering of such securities at that time shall be deemed to be in the initial
  bona fide offering thereof.
  
          3.      Insofar as indemnification for liabilities arising under the
  Securities Act may be permitted to directors, officers and controlling persons
  of the Registrant pursuant to the foregoing provisions, or otherwise, the
  Registrant has been advised that in the opinion of the Securities and Exchange
  Commission such indemnification is against public policy as expressed in the
  Securities Act and is, therefore, unenforceable. In the event that a claim for
  indemnification against such liabilities (other than the payment by the
  Registrant of expenses incurred or paid by a director, officer or controlling
  person of the Registrant in the successful defense of any action, suit or
  proceeding) is asserted by such director, officer or controlling person in
  connection with the securities being registered, the Registrant will, unless
  in the opinion of its counsel the matter has been settled by controlling
  precedent, submit to a court of appropriate jurisdiction the question whether
  such indemnification by it is against public policy as expressed in the
  Securities Act and will be governed by the final adjudication of such issue.
  
  
  
  
  
  
  <PAGE>   3
                                     SIGNATURES
  
        Pursuant to the requirements of the Securities Act of 1933, the Registrant
  certifies that it has reasonable grounds to believe that it meets all of the
  requirements for filing on Form S-8 and has duly caused this Registration
  Statement to be signed on its behalf by the undersigned, thereunto duly
  authorized, in the Town of Marlborough, Commonwealth of Massachusetts on the 5th
  day of June, 1996.
  
                                                   SEPRACOR INC.
  
                                                   By: /s/ Timothy J. Barberich
                                                       ------------------------
                                                       Timothy J. Barberich
                                                       President and
                                                       Chief Executive Officer
  
                                  POWER OF ATTORNEY
  
        We, the undersigned officers and directors of Sepracor Inc. hereby
  severally constitute Timothy J. Barberich and Susan W. Murley, Esq. and each of
  them singly, our true and lawful attorneys with full power to them, and each of
  them singly, to sign for us and in our names in the capacities indicated below,
  the Registration Statement on Form S-8 filed herewith and any and all subsequent
  amendments to said Registration Statement, and generally to do all such things
  in our names and behalf in our capacities as officers and directors to enable
  Sepracor Inc. to comply with all requirements of the Securities and Exchange
  Commission, hereby ratifying and confirming our signatures as they may be signed
  by said attorneys, or any of them, to said Registration Statement and any and
  all amendments thereto.
  
                                                                            
                                        -3-
  <PAGE>   4
        Pursuant to the requirements of the Securities Act of 1933, this
  Registration Statement has been signed below by the following persons in the
  capacities and on the date indicated.
  
  Signature                          Title                         Date
  - ---------                          -----                         ----
  
  /s/ Timothy J. Barberich
  - -------------------------          President, Chief         )
  Timothy J. Barberich               Executive Officer        )
                                     and Director             )
                                     (Principal Executive     )
                                      Officer)                )
                                                              )
  /s/ David P. Southwell                                      )
  - -------------------------          Chief Financial          )
  David P. Southwell                 Officer (Principal       )
                                     Financial Officer)       )
                                                              )
  /s/ Robert F. Scumaci                                       )
  - -------------------------          Senior Vice President,   )
  Robert F. Scumaci                  Finance and              )
                                     Administration and       )
                                     Treasurer                )
                                     (Principal Accounting    )
                                               Officer)       )
                                                              )
                                                              )
  /s/ James G. Andress               Director                 )
  - -------------------------                                   )
  James G. Andress                                            )
                                                              )
                                                              )
                                                              ) June 5, 1996
                                                              )
  /s/ Digby W. Barrios               Director                 )
  - -------------------------                                   )
  Digby W. Barrios                                            )
                                                              )
                                                              )
                                                              )
  /s/ Robert J. Cresci               Director                 )
  - -------------------------                                   )
  Robert J. Cresci                                            )
                                                              )
                                                              )
                                                              )
  /s/ Robert F. Johnston             Director                 )
  - -------------------------                                   )
  Robert F. Johnston                                          )
                                                              )
                                                              )
                                                              )
  /s/ Keith Mansford                 Director                 )
  - -------------------------                                   )
  Keith Mansford                                              )
                                                              )
                                                              )
                                                              )
  /s/ James F. Mrazek                Director                 )
  - -------------------------                                   )
  James F. Mrazek                                             )
                                                              )
                                                              )
                                                              )
  /s/ Alan A. Steigrod               Director                 )
  - -------------------------                                   )
  Alan A. Steigrod                                            ) 
                                                    
                                        -4-
  <PAGE>   5
                                    EXHIBIT INDEX
  
  Exhibit
  Number         Description
  - ------         -----------
   4.1           Restated Certificate of Incorporation of the Registrant, as 
                 amended
  
   4.2 *         Amended and Restated By-Laws of the Registrant, as amended
  
   4.3 *         Specimen Certificate of Common Stock of the Registrant
  
     5           Opinion of Hale and Dorr
  
  23.1           Consent of Hale and Dorr (included in Exhibit 5)
  
  23.2           Consent of Coopers & Lybrand L.L.P.
  
    24           Power of Attorney (included in the signature pages of this 
                 Registration Statement)
  
  - -------------------------                                   
  *     Incorporated herein by reference from the Registrant's Registration
        Statement on Form S-1 (File No. 33-41653).
  
                                         -5-
metadata_documents: [{"type": "S-8", "sequence": "1", "description": "SEPRACOR, INC. 1991 DIRECTOR STOCK OPTION PLAN", "secsgml_size_bytes": 15845}]
metadata_filer: {"company-data": {"conformed-name": "SEPRACOR INC /DE/", "cik": "0000877357", "assigned-sic": "3826", "irs-number": "222536587", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-05217", "film-number": "96576911"}, "business-address": {"street1": "33 LOCKE DRIVE", "city": "MARLBOROUGH", "state": "MA", "zip": "01752", "phone": "5084816700"}}
text: 1
                As filed with the Securities and Exchange Commission
                                   on June 5, 1996
  
  ================================================================================
  
                         SECURITIES AND EXCHANGE COMMISSION
                               Washington, D.C. 20549
  
                                      FORM S-8
  
                            REGISTRATION STATEMENT UNDER
                             THE SECURITIES ACT OF 1933
  
                                    SEPRACOR INC.
               (Exact name of registrant as specified in its charter)
  
             DELAWARE                                           22-2536587
  (State or other jurisdiction of                            (I.R.S. Employer
   incorporation or organization)                           Identification No.)
  
                  111 LOCKE DRIVE, MARLBOROUGH, MASSACHUSETTS   01752
                 (Address of Principal Executive Offices)    (Zip Code)
  
                           1991 DIRECTOR STOCK OPTION PLAN
                              (Full title of the plan)
  
                                TIMOTHY J. BARBERICH
                                    SEPRACOR INC.
                                   111 LOCKE DRIVE
                          MARLBOROUGH, MASSACHUSETTS 01752
                       (Name and address of agent for service)
  
                                   (508) 481-6700
            (Telephone number, including area code, of agent for service)
  
                           CALCULATION OF REGISTRATION FEE
  Proposed    Proposed
  Title of                    maximum     maximum
  securities     Amount       offering    aggregate       Amount of
  to be          to be        price       offering        registration
  registered     registered   per share   price           fee
  - ----------     ----------   ---------   ------------    ------------
  Common Stock,   75,000      $14.44(1)   $1,083,000(1)     $374.00
  $.10 par        shares
  value
  - --------------------------------------------------------------------------------
  
  (1)   Estimated solely for the purpose of calculating the registration fee, and
        based upon the average of the high and low prices of the Common Stock on
        the Nasdaq National Market on May 30, 1996 in accordance with Rules 457(c)
        and 457(h) of the Securities Act of 1933.
  
  ================================================================================
  2
  Statement of Incorporation by Reference
  
          Except as otherwise set forth below, this Registration Statement on 
  Form S-8 incorporates by reference the contents of the Registration Statement 
  on Form S-8, File No. 33-44808, relating to the Registrant's 1991 Director 
  Stock Option Plan (the "Initial Registration Statement").
  
  PART II. INFORMATION REQUIRED IN THE REGISTRATION STATEMENT
  
          Item 3 of the Initial Registration Statement is amended and restated
  in its entirety as follows:
  
          Item 3.   Incorporation of Certain Documents by Reference.
                    -----------------------------------------------
  
          The Registrant is subject to the informational and reporting
  requirements of Sections 13(a), 14, and 15(d) of the Securities Exchange Act of
  1934 (as amended, the "Exchange Act"), and in accordance therewith files
  reports, proxy statements and other information with the Securities and
  Exchange Commission.  The following documents, which are filed with the
  Securities and Exchange Commission, are incorporated in this Prospectus by
  reference.
  
                  (1) The Registrant's latest annual report filed pursuant to
          Section 13(a) or 15(d) of the Exchange Act, or the latest prospectus
          filed pursuant to Rule 424(b) under the Securities Act of 1933 (as
          amended, the "Securities Act"), that contains audited financial
          statements for the Registrant's latest fiscal year for which such
          statements have been filed.
  
                  (2) All other reports filed pursuant to Section 13(a) or 15(d)
          of the Exchange Act since the end of the fiscal year covered by the
          document referred to in (1) above.
  
                  (3) The description of the Common Stock, par value $.10 per
          share ("Common Stock"), contained in a registration statement filed
          under the Exchange Act, including any amendment or report filed for the
          purpose of updating such description.
  
          All documents subsequently filed by the Registrant pursuant to Sections
  13(a), 13(c), 14, and 15(d) of the Exchange Act prior to the filing of a
  post-effective amendment which indicates that all shares of Common Stock 
  offered hereby have been sold or which deregisters all shares of Common Stock 
  then remaining unsold, shall be deemed to be incorporated by reference herein
  and to be part hereof from the date of the filing of such documents.
  
          Item 9 of the Initial Registration Statement is amended and restated in
  its entirety as follows:
  
          Item 9.  Undertakings.
                   ------------
  
          1.   The Company hereby undertakes.
  
               (a) To file, during any period in which offers or sales are being
          made, a post-effective amendment to this registration statement:
  
               (i) To include any prospectus required by Section 10(a)(3) of the
          Securities Act;
  
               (ii) To reflect in the prospectus any facts or events arising
          after the effective date of the registration statement (or the most
          recent post-effective amendment thereof) which, individually or in the
          aggregate, represent a fundamental change in the information set forth
          in the registration statement; and
  
               (iii) To include any material information with respect to the plan
          of distribution not previously disclosed in the registration statement
          or any material change to such information in the registration
          statement;
  
          PROVIDED, HOWEVER that paragraphs (i) and (ii) do not apply if the
          registration statement is on Form S-3 or Form S-8, and the information
          required to be included in a post-effective amendment by those
          paragraphs is contained in periodic reports filed by the Company
          pursuant to Section 13 or Section 15(d) of the Exchange Act that are
          incorporated by reference in the registration statement.
  
                  (b) That, for the purpose of determining any liability under
          the Securities Act, each such post-effective amendment shall be deemed
          to be a new registration statement relating to the securities offered
          therein, and the offering of such securities at that time shall be
          deemed to be the initial bona fide offering thereof.
  
                  (c) To remove from registration by means of a post-effective
          amendment any of the securities being registered which remain unsold at
          the termination of the offering.
  
          2.      The Company hereby undertakes that, for purposes of determining
  any liability under the Securities Act, each filing of the Company's annual
  report pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and,
  where applicable, each filing of an employee benefit plan's annual report
  pursuant to Section 15(d) of the Exchange Act) that is incorporated by
  reference in the registration statement shall be deemed to be a new
  registration statement relating to the securities offered therein, and the
  offering of such securities at that time shall be deemed to be in the initial
  bona fide offering thereof.
  
          3.      Insofar as indemnification for liabilities arising under the
  Securities Act may be permitted to directors, officers and controlling persons
  of the Registrant pursuant to the foregoing provisions, or otherwise, the
  Registrant has been advised that in the opinion of the Securities and Exchange
  Commission such indemnification is against public policy as expressed in the
  Securities Act and is, therefore, unenforceable. In the event that a claim for
  indemnification against such liabilities (other than the payment by the
  Registrant of expenses incurred or paid by a director, officer or controlling
  person of the Registrant in the successful defense of any action, suit or
  proceeding) is asserted by such director, officer or controlling person in
  connection with the securities being registered, the Registrant will, unless
  in the opinion of its counsel the matter has been settled by controlling
  precedent, submit to a court of appropriate jurisdiction the question whether
  such indemnification by it is against public policy as expressed in the
  Securities Act and will be governed by the final adjudication of such issue.
  3
                                     SIGNATURES
  
        Pursuant to the requirements of the Securities Act of 1933, the Registrant
  certifies that it has reasonable grounds to believe that it meets all of the
  requirements for filing on Form S-8 and has duly caused this Registration
  Statement to be signed on its behalf by the undersigned, thereunto duly
  authorized, in the Town of Marlborough, Commonwealth of Massachusetts on the 5th
  day of June, 1996.
  
                                                   SEPRACOR INC.
  
                                                   By: /s/ Timothy J. Barberich
                                                       ------------------------
                                                       Timothy J. Barberich
                                                       President and
                                                       Chief Executive Officer
  
                                  POWER OF ATTORNEY
  
        We, the undersigned officers and directors of Sepracor Inc. hereby
  severally constitute Timothy J. Barberich and Susan W. Murley, Esq. and each of
  them singly, our true and lawful attorneys with full power to them, and each of
  them singly, to sign for us and in our names in the capacities indicated below,
  the Registration Statement on Form S-8 filed herewith and any and all subsequent
  amendments to said Registration Statement, and generally to do all such things
  in our names and behalf in our capacities as officers and directors to enable
  Sepracor Inc. to comply with all requirements of the Securities and Exchange
  Commission, hereby ratifying and confirming our signatures as they may be signed
  by said attorneys, or any of them, to said Registration Statement and any and
  all amendments thereto.
  
                                                                            
                                        -3-
  4
        Pursuant to the requirements of the Securities Act of 1933, this
  Registration Statement has been signed below by the following persons in the
  capacities and on the date indicated.
  
  Signature                          Title                         Date
  - ---------                          -----                         ----
  
  /s/ Timothy J. Barberich
  - -------------------------          President, Chief         )
  Timothy J. Barberich               Executive Officer        )
                                     and Director             )
                                     (Principal Executive     )
                                      Officer)                )
                                                              )
  /s/ David P. Southwell                                      )
  - -------------------------          Chief Financial          )
  David P. Southwell                 Officer (Principal       )
                                     Financial Officer)       )
                                                              )
  /s/ Robert F. Scumaci                                       )
  - -------------------------          Senior Vice President,   )
  Robert F. Scumaci                  Finance and              )
                                     Administration and       )
                                     Treasurer                )
                                     (Principal Accounting    )
                                               Officer)       )
                                                              )
                                                              )
  /s/ James G. Andress               Director                 )
  - -------------------------                                   )
  James G. Andress                                            )
                                                              )
                                                              )
                                                              ) June 5, 1996
                                                              )
  /s/ Digby W. Barrios               Director                 )
  - -------------------------                                   )
  Digby W. Barrios                                            )
                                                              )
                                                              )
                                                              )
  /s/ Robert J. Cresci               Director                 )
  - -------------------------                                   )
  Robert J. Cresci                                            )
                                                              )
                                                              )
                                                              )
  /s/ Robert F. Johnston             Director                 )
  - -------------------------                                   )
  Robert F. Johnston                                          )
                                                              )
                                                              )
                                                              )
  /s/ Keith Mansford                 Director                 )
  - -------------------------                                   )
  Keith Mansford                                              )
                                                              )
                                                              )
                                                              )
  /s/ James F. Mrazek                Director                 )
  - -------------------------                                   )
  James F. Mrazek                                             )
                                                              )
                                                              )
                                                              )
  /s/ Alan A. Steigrod               Director                 )
  - -------------------------                                   )
  Alan A. Steigrod                                            ) 
                                                    
                                        -4-
  5
                                    EXHIBIT INDEX
  
  Exhibit
  Number         Description
  - ------         -----------
   4.1           Restated Certificate of Incorporation of the Registrant, as 
                 amended
  
   4.2 *         Amended and Restated By-Laws of the Registrant, as amended
  
   4.3 *         Specimen Certificate of Common Stock of the Registrant
  
     5           Opinion of Hale and Dorr
  
  23.1           Consent of Hale and Dorr (included in Exhibit 5)
  
  23.2           Consent of Coopers & Lybrand L.L.P.
  
    24           Power of Attorney (included in the signature pages of this 
                 Registration Statement)
  
  - -------------------------                                   
  *     Incorporated herein by reference from the Registrant's Registration
        Statement on Form S-1 (File No. 33-41653).
  
                                         -5-