<PAGE>


                 DEVELOPMENT AND COMMERCIALIZATION AGREEMENT (NO. 1)



                                       BETWEEN



                                   COCENSYS, INC.,


                                         AND


                              WYETH-AYERST LABORATORIES

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                                  TABLE OF CONTENTS

                                                                          PAGE

1.  DEFINITIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   1
    1.1   "Affiliate(s)" . . . . . . . . . . . . . . . . . . . . . . . . .   1
    1.2   "Agreement (No. 2)"  . . . . . . . . . . . . . . . . . . . . . .   1
    1.3   "All Other Markets". . . . . . . . . . . . . . . . . . . . . . .   2
    1.4   "Back-Up Compound Candidate" . . . . . . . . . . . . . . . . . .   2
    1.5   "Back-Up Program". . . . . . . . . . . . . . . . . . . . . . . .   2
    1.6   "Back-Up Program Term" . . . . . . . . . . . . . . . . . . . . .   2
    1.7   "Beginning of [     *     ]. . . . . . . . . . . . . . . . . . .   2
    1.8   "Beginning of [     *     ]. . . . . . . . . . . . . . . . . . .   2
    1.9   "CO 2-6749". . . . . . . . . . . . . . . . . . . . . . . . . . .   2
    1.10  "CoCensys Patent Rights" . . . . . . . . . . . . . . . . . . . .   2
    1.11  "Commercially Reasonable Efforts". . . . . . . . . . . . . . . .   2
    1.12  "Control". . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
    1.13  "Co-Promotion" . . . . . . . . . . . . . . . . . . . . . . . . .   2
    1.14  "Development". . . . . . . . . . . . . . . . . . . . . . . . . .   3
    1.15  "Epalon" . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
    1.16  "FDA". . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
    1.17  "Field". . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
    1.18  "First Commercial Sale". . . . . . . . . . . . . . . . . . . . .   3
    1.19  "Fully Burdened Cost". . . . . . . . . . . . . . . . . . . . . .   3
    1.20  "Good Clinical Practice" . . . . . . . . . . . . . . . . . . . .   3
    1.21  "Good Laboratory Practice" . . . . . . . . . . . . . . . . . . .   3
    1.22  "Good Manufacturing Practice". . . . . . . . . . . . . . . . . .   4
    1.23  "Gross Margin" . . . . . . . . . . . . . . . . . . . . . . . . .   4
    1.24  [     *     ]. . . . . . . . . . . . . . . . . . . . . . . . . .   4
    1.25  "IND". . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
    1.26  "Intellectual Property Rights" . . . . . . . . . . . . . . . . .   4
    1.27  "Invention". . . . . . . . . . . . . . . . . . . . . . . . . . .   4
    1.28  "Joint Marketing Committee" or "JMC" . . . . . . . . . . . . . .   4
    1.29  "Know-How" . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
    1.30  "Licensed Compound". . . . . . . . . . . . . . . . . . . . . . .   5
    1.31  "Marketing Plan" . . . . . . . . . . . . . . . . . . . . . . . .   5
    1.32  "NDA". . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
    1.33  "Net Sales". . . . . . . . . . . . . . . . . . . . . . . . . . .   5
    1.34  "Neurologist Market" . . . . . . . . . . . . . . . . . . . . . .   6
    1.35  [     *     ]. . . . . . . . . . . . . . . . . . . . . . . . . .   6
    1.36  "Non-Scheduled Product". . . . . . . . . . . . . . . . . . . . .   6
    1.37  "Patent Rights". . . . . . . . . . . . . . . . . . . . . . . . .   6
    1.38  "Phase I," . . . . . . . . . . . . . . . . . . . . . . . . . . .   6


                                          i.
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                                  TABLE OF CONTENTS
                                     (CONTINUED)

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    1.39  "Prescription Audit" . . . . . . . . . . . . . . . . . . . . . .   6
    1.40  "Product". . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
    1.41  "Promotion". . . . . . . . . . . . . . . . . . . . . . . . . . .   6
    1.42  "Psychiatrist Market". . . . . . . . . . . . . . . . . . . . . .   7
    1.43  "Regulatory Approval". . . . . . . . . . . . . . . . . . . . . .   7
    1.44  "Scheduled Product". . . . . . . . . . . . . . . . . . . . . . .   7
    1.45  "Term of Co-Promotion" . . . . . . . . . . . . . . . . . . . . .   7
    1.46  "Third Party(ies)" . . . . . . . . . . . . . . . . . . . . . . .   7
    1.47  "Trademark". . . . . . . . . . . . . . . . . . . . . . . . . . .   7
    1.48  "USA". . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
    1.49  "Wyeth-Ayerst Patent Rights" . . . . . . . . . . . . . . . . . .   7

2.  DEVELOPMENT OF LICENSED COMPOUND . . . . . . . . . . . . . . . . . . .   7
    2.1   General Obligations. . . . . . . . . . . . . . . . . . . . . . .   7
    2.2   Records, Reports and Information Exchange. . . . . . . . . . . .   8
    2.3   Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
    2.4   Funding of the Development . . . . . . . . . . . . . . . . . . .   8
    2.5   Regulatory Approvals . . . . . . . . . . . . . . . . . . . . . .   8

3.  BACK-UP PROGRAM. . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
    3.1   Purpose. . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
    3.2   Back-Up Program Term . . . . . . . . . . . . . . . . . . . . . .   9
    3.3   Reporting and Submission of Back-Up Compound Candidates. . . . .   9
    3.4   Back-Up Compound Development.. . . . . . . . . . . . . . . . . .   9
    3.5   Back-Up Program Funding. . . . . . . . . . . . . . . . . . . . .   9
    3.6   Partial Reimbursement of Back-Up Program Fees. . . . . . . . . .   9
    3.7   Applicability of this Agreement. . . . . . . . . . . . . . . . .  10

4.  INDICATIONS; EXCLUSIVITY . . . . . . . . . . . . . . . . . . . . . . .  10
    4.1   Indications. . . . . . . . . . . . . . . . . . . . . . . . . . .  10
    4.2   No [     *     ] by CoCensys [     *     ] . . . . . . . . . . .  11
    4.3   No [     *     ] . . . . . . . . . . . . . . . . . . . . . . . .  11

5.  LICENSE GRANTS . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
    5.1   Licenses to Wyeth-Ayerst . . . . . . . . . . . . . . . . . . . .  11
    5.2   Licenses to CoCensys . . . . . . . . . . . . . . . . . . . . . .  11
    5.3   Licenses upon Expiration . . . . . . . . . . . . . . . . . . . .  12


                                         ii.

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                                  TABLE OF CONTENTS
                                     (CONTINUED)

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6.  CONSIDERATION TO COCENSYS. . . . . . . . . . . . . . . . . . . . . . .  12
    6.1   Upfront Fee. . . . . . . . . . . . . . . . . . . . . . . . . . .  12
    6.2   Equity Investment. . . . . . . . . . . . . . . . . . . . . . . .  12
    6.3   Payments . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
    6.4   Method of Payment of Payments. . . . . . . . . . . . . . . . . .  13
    6.5   Payments for Back-Up Compound Candidates . . . . . . . . . . . .  13
    6.6   Royalties and Profit Sharing . . . . . . . . . . . . . . . . . .  13

7.  CO-PROMOTION OF THE PRODUCT IN THE USA . . . . . . . . . . . . . . . .  13
    7.1   Promotion and Co-Promotion Rights. . . . . . . . . . . . . . . .  13
    7.2   Joint Marketing Committee. . . . . . . . . . . . . . . . . . . .  16
    7.3   Marketing Plan . . . . . . . . . . . . . . . . . . . . . . . . .  18
    7.4   Sales Force Expenses . . . . . . . . . . . . . . . . . . . . . .  18
    7.5   Determination and Allocation of Gross Margin and Royalties . . .  18
    7.6   Adjustment to Royalties. . . . . . . . . . . . . . . . . . . . .  19
    7.7   Prescription Audit.. . . . . . . . . . . . . . . . . . . . . . .  20
    7.8   Payment and Reporting. . . . . . . . . . . . . . . . . . . . . .  20
    7.9   Promotional and Advertising Materials. . . . . . . . . . . . . .  21
    7.10  Pricing. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
    7.11  No Delegation. . . . . . . . . . . . . . . . . . . . . . . . . .  21
    7.12  Returns. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
    7.13  Orders . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
    7.14  Samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
    7.15  Completion of Sales. . . . . . . . . . . . . . . . . . . . . . .  22
    7.16  Training . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
    7.17  Exchange of Marketing Information. . . . . . . . . . . . . . . .  22

8.  ACCOUNTS AND RECORDS . . . . . . . . . . . . . . . . . . . . . . . . .  22
    8.1   Records. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
    8.2   Audits.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
    8.3   Sales by Sublicensees. . . . . . . . . . . . . . . . . . . . . .  23
    8.4   Withholding. . . . . . . . . . . . . . . . . . . . . . . . . . .  23

9.  TRADEMARKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  23

10. MANUFACTURING AND DISTRIBUTION . . . . . . . . . . . . . . . . . . . .  23
    10.1  Primary Manufacture. . . . . . . . . . . . . . . . . . . . . . .  23
    10.2  Second Source. . . . . . . . . . . . . . . . . . . . . . . . . .  23
    10.3  Exchange of Information. . . . . . . . . . . . . . . . . . . . .  24


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                                  TABLE OF CONTENTS
                                     (CONTINUED)

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    10.4  Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . .  24

11. PROSECUTION, MAINTENANCE AND INFRINGEMENT OF INTELLECTUAL PROPERTY
    RIGHTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
    11.1  Patentable Inventions. . . . . . . . . . . . . . . . . . . . . .  24
    11.2  Prosecution and Maintenance of Patent Rights . . . . . . . . . .  25
    11.3  Patent Extensions. . . . . . . . . . . . . . . . . . . . . . . .  25
    11.4  Cooperation. . . . . . . . . . . . . . . . . . . . . . . . . . .  25
    11.5  Infringement of Intellectual Property Rights . . . . . . . . . .  25
    11.6  Infringement of Third Party Patent Rights. . . . . . . . . . . .  27

12. FORCE MAJEURE. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28

13. TERM AND TERMINATION . . . . . . . . . . . . . . . . . . . . . . . . .  28
    13.1  General Conditions of Expiration and Termination . . . . . . . .  28
    13.2  Term . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
    13.3  Termination for Breach . . . . . . . . . . . . . . . . . . . . .  29
    13.4  Breach by CoCensys . . . . . . . . . . . . . . . . . . . . . . .  29
    13.5  No Limit on Remedies . . . . . . . . . . . . . . . . . . . . . .  29
    13.6  Unilateral Termination by Wyeth-Ayerst . . . . . . . . . . . . .  29
    13.7  Determination of Co-Promotion Rights upon Change in Control. . .  30

14. ASSIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
    14.1  Assignment to Affiliates . . . . . . . . . . . . . . . . . . . .  30
    14.2  Other Permitted Assignment . . . . . . . . . . . . . . . . . . .  30
    14.3  Binding Nature of Assignment . . . . . . . . . . . . . . . . . .  31

15. INDEMNIFICATION. . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
    15.1  Cross Indemnification. . . . . . . . . . . . . . . . . . . . . .  31
    15.2  Insurance. . . . . . . . . . . . . . . . . . . . . . . . . . . .  31

16. WARRANTIES AND REPRESENTATIONS . . . . . . . . . . . . . . . . . . . .  32
    16.1  General. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
    16.2  USC/Rockefeller License. . . . . . . . . . . . . . . . . . . . .  32

17. CONFIDENTIAL INFORMATION . . . . . . . . . . . . . . . . . . . . . . .  32
    17.1  Information. . . . . . . . . . . . . . . . . . . . . . . . . . .  32
    17.2  Exceptions.. . . . . . . . . . . . . . . . . . . . . . . . . . .  32
    17.3  Permitted Disclosures. . . . . . . . . . . . . . . . . . . . . .  33


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    17.4  Disclosure of Agreement. . . . . . . . . . . . . . . . . . . . .  33
    17.5  Publicity. . . . . . . . . . . . . . . . . . . . . . . . . . . .  33
    17.6  Publication. . . . . . . . . . . . . . . . . . . . . . . . . . .  34

18. MISCELLANEOUS. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
    18.1  No Waiver of Contractual Rights. . . . . . . . . . . . . . . . .  34
    18.2  Execution and Amendments . . . . . . . . . . . . . . . . . . . .  34
    18.3  Severability . . . . . . . . . . . . . . . . . . . . . . . . . .  34
    18.4  Relationship between the Parties . . . . . . . . . . . . . . . .  34
    18.5  Correspondence and Notices . . . . . . . . . . . . . . . . . . .  35
    18.6  Choice of Law. . . . . . . . . . . . . . . . . . . . . . . . . .  35
    18.7  Counterparts . . . . . . . . . . . . . . . . . . . . . . . . . .  36


                                          v.
<PAGE>

                 DEVELOPMENT AND COMMERCIALIZATION AGREEMENT (NO. 1)


     THIS DEVELOPMENT AND COMMERCIALIZATION AGREEMENT (NO. 1) (the "Agreement")
is entered into as of the 12th day of May, 1997 (the "Effective Date"), by and
between COCENSYS, INC., a company incorporated under the laws of the State of
Delaware, with its principal place of business at 213 Technology Drive, Irvine,
California 92618, USA ("CoCensys"), and AMERICAN HOME PRODUCTS CORPORATION,
acting through its WYETH-AYERST LABORATORIES DIVISION, a company incorporated
under the laws of the State of Delaware, with its principal place of business at
555 Lancaster Avenue, St. Davids, Pennsylvania 19087, USA ("Wyeth-Ayerst"). 
Both CoCensys and Wyeth-Ayerst are referred to individually as a "Party" and
collectively as the "Parties."

     WHEREAS, CoCensys has discovered, has rights to and is developing that
certain compound CO 2-6749 (as defined in Article 1.9 below); and

     WHEREAS, Wyeth-Ayerst would like to obtain the United States rights to
develop and commercialize CO 2-6749 or a back-up compound therefor; and

     WHEREAS, CoCensys and Wyeth-Ayerst wish to co-promote CO 2-6749 or a
back-up compound in the United States;

     NOW, THEREFORE, in consideration of the foregoing premises and the mutual
premises, covenants and conditions contained in this Agreement, the Parties
agree as follows:

1.   DEFINITIONS.

     For the purposes of this Agreement, the terms hereunder shall have the
     meanings as defined below:

     1.1   "AFFILIATE(S)" shall mean, in the case of either CoCensys or
           Wyeth-Ayerst, any corporation, joint venture, or other business
           entity which directly or indirectly controls, is controlled by, or
           is under common control with that Party.  "Control," as used in this
           Article 1.1, shall mean having the power to direct, or cause the
           direction of, the management and policies of an entity, whether
           through ownership of voting securities, by contract or otherwise.

     1.2   "AGREEMENT (NO. 2)" shall mean that certain Development and
           Commercialization Agreement (No.2) between the Parties relating to
           development and commercialization of CO 2-6749 or a back-up compound
           therefore in all countries and territories in the world except the
           USA, entered into on even date with the Effective Date.


                                          1.
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     1.3   "ALL OTHER MARKETS" shall mean all physicians, other than
           neurologists and psychiatrists, prescribing pharmaceutical products
           in the USA during the Term of Co-Promotion, including without
           limitation, all general practitioners.
     
     1.4   "BACK-UP COMPOUND CANDIDATE" shall mean any Epalon meeting the
           criteria set forth in Exhibit A as determined by CoCensys and
           submitted by CoCensys to Wyeth-Ayerst as a potential substitute for
           CO 2-6749.  "Back-Up Compound Candidate" shall include any prodrugs,
           salt forms and other biologically active isomers or enantiomers of
           such CoCensys Epalon.

     1.5   "BACK-UP PROGRAM" shall mean the program for identification of
           Back-Up Compound Candidates as conducted by CoCensys pursuant to
           Article 3.

     1.6   "BACK-UP PROGRAM TERM" shall mean the period commencing on the
           Effective Date and terminating on the [     *     ] anniversary
           thereof, unless extended pursuant to Article 3.2.

     1.7   "BEGINNING OF [     *     ] shall mean the date upon which the
           [     *     ] for a Product.

     1.8   "BEGINNING OF [     *     ] shall mean the date upon which the
           [     *     ] for a Product.

     1.9   "CO 2-6749"  shall mean the Epalon [     *     ] and any prodrugs,
           including specifically CO 6-0549, salt forms and other biologically
           active isomers or enantiomers palon.  A diagram of the chemical
           structure of each of CO 2-6749 and CO 6-0549 is set forth on Exhibit
           B.

     1.10  "COCENSYS PATENT RIGHTS" shall mean all Patent Rights owned or
           Controlled by CoCensys.  A list of the CoCensys Patent Rights
           existing as of the Effective Date is set out in Exhibit C hereto.

     1.11  "COMMERCIALLY REASONABLE EFFORTS" shall mean efforts and resources
           normally used by a party for a compound owned by it or to which it
           has rights, which is of similar market potential at a similar stage
           in its product life, taking into account the competitiveness of the
           marketplace, the proprietary position of the compound, the
           regulatory structure involved, the profitability of the applicable
           products, and other relevant factors.

     1.12  "CONTROL" shall mean licensed with the right to grant sublicenses
           without violating the terms of any Third Party agreement.


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     1.13  "CO-PROMOTION" shall mean the promotion, marketing and selling of
           the Product, including, in particular, the detailing of the Product
           to physicians, jointly through the sales forces of CoCensys and
           Wyeth-Ayerst in the USA under the Trademark and Regulatory Approval
           held by Wyeth-Ayerst and Wyeth-Ayerst's distribution system.

     1.14  "DEVELOPMENT" shall mean the pre-clinical development of a Licensed
           Compound and clinical development for use in the Field of a Product
           through and including Regulatory Approval.

     1.15  "EPALON" shall mean the class of neuroactive steroid compounds that
           interact with GABA(A) receptor complexes.

     1.16  "FDA" shall mean the Food and Drug Administration of the USA.

     1.17  "FIELD" shall mean the treatment in humans of 
           [          *          ] in the Diagnostic and Statistical Manual of 
           Mental Disorders, Fourth Edition ("DSM-IV").  Also, specifically to 
           be included under [          *          ] defined by research 
           criteria in DSM-IV. It is understood by the Parties that the above-
           named disorders may d differently in future editions of the 
           Diagnostic and Statistical Manual of Mental Disorders, but that, 
           for purposes of this Agreement, the definition of Field shall 
           always be with reference to those diseases which fall within the 
           definition of the above-named disorders in the edition of DSM-IV 
           which is current as of the Effective Date.  Expressly excluded from 
           the Field are [          *          ].

     1.18  "FIRST COMMERCIAL SALE" shall mean the first sale of a Product in
           the USA after the Product has been granted Regulatory Approval by
           the competent authorities in the USA.

     1.19  "FULLY BURDENED COST" shall mean the sum of the costs set forth in
           Exhibit D, to the extent allocable to Product.

     1.20  "GOOD CLINICAL PRACTICE" or "GCP" shall mean the then current
           standards for clinical trials for pharmaceuticals, as set forth in
           the United States Federal Food, Drug and Cosmetics Act and
           applicable regulations promulgated thereunder, as amended from time
           to time.

     1.21  "GOOD LABORATORY PRACTICE" or "GLP" shall mean the then current
           standards for laboratory activities for pharmaceuticals, as set
           forth in the United States Federal Food, Drug and Cosmetics Act and
           applicable regulations promulgated thereunder, as amended from time
           to time.


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     1.22  "GOOD MANUFACTURING PRACTICE" or "GMP" shall mean the current
           standards for the manufacture of pharmaceuticals, as set forth in
           the United States Federal Food, Drug and Cosmetics Act and
           applicable regulations promulgated hereunder, as amended from time
           to time.

     1.23  "GROSS MARGIN" shall mean Net Sales in either the Neurologist
           Market, the Psychiatrist Market and/or All Other Markets, as the
           case may be, minus the Fully Burdened Cost for such Net Sales.

     1.24  [          *          ] shall mean the development milestone 
           [          *          ].

     1.25  "IND" shall mean an Investigational New Drug Application as defined
           in the United States Federal Food, Drug and Cosmetics Act and
           applicable regulations promulgated thereunder, as amended from time
           to time.

     1.26  "INTELLECTUAL PROPERTY RIGHTS" shall mean all Patent Rights,
           trademarks, copyrights, know-how and/or trade secrets which are
           owned or Controlled by one Party hereto (with the right to license)
           or jointly by the Parties, with regard to the development,
           manufacture, importing, use, marketing and/or sale of the Product.

     1.27  "INVENTION" shall mean an invention conceived in the course of the
           performance of and within the scope of this Agreement.

     1.28  "JOINT MARKETING COMMITTEE" or "JMC" shall mean the committee
           appointed by the Parties as set forth in Article 7.2.

     1.29  "KNOW-HOW" shall mean all know-how, processes, information and data
           including any copyright relating thereto owned or controlled by
           either Party (with the right to have or disclose) as of the
           Effective Date or acquired during the term of this Agreement
           relating to:

           (a) the Licensed Compound;

           (b) any Back-Up Compound;

           (c) any Product containing (a) or (b);

           (d) methods of making any of (a), (b) or (c);

           (e) any component of (c);

           (f) any intermediate in the making of any of (a), (b), (c), or
               (e);


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         (G)  any method of using any of (a), (b), (c), (e), or (f); and/or

         (H)  any use of (a), (b) or (c).

         The term "Know-How," however, shall not include any know-how,
         processes, information and data which is, as of the Effective Date or
         becomes later on, generally available to the public.

    1.30 "LICENSED COMPOUND" shall mean CO 2-6749 and any Back-Up Compound
         Candidate.  

    1.31 "MARKETING PLAN" shall have the meaning set forth in Article 7.3.

    1.32 "NDA" shall mean a New Drug Application, as defined in the United
         States Federal Food Drug and Cosmetic Act and applicable regulations
         promulgated thereunder as amended from time to time.

    1.33 "NET SALES" shall mean proceeds from sales of the Product by either
         Party, its Affiliates or sublicensees, as appropriate, to Third
         Parties, less the sum of (a) and (b) where (a) is a provision,
         determined under generally accepted accounting principles in the
         United States, for [          *          ] and (b) is [          *     
             ]

         Sales of Product by and between a Party and its Affiliates are not
         sales to Third Parties and shall be excluded from Net Sales
         calculations for all purposes.

    1.34 "NEUROLOGIST MARKET" shall mean all neurologists prescribing
         pharmaceutical products in the USA during the Term of Co-Promotion.

    1.35  [     *     ] shall have the meaning set forth in Article 4.1.

    1.36 "NON-SCHEDULED PRODUCT" shall mean a Product that has achieved
         Regulatory Approval in the USA as either a "Schedule V" designation,
         as defined in 21 CFR Part 1308, or no scheduling designation under 21
         CFR Part 1308.

    1.37 "PATENT RIGHTS" shall mean all patents or patent applications, in the
         USA, and all divisionals, continuations, continuations-in-part,
         reissues, extensions, supplementary protection certificates thereof,
         existing as of the Effective Date or filed or issuing during the term
         of this Agreement, at least one claim of which covers:

         (A)  the Licensed Compound;

         (B)  any Back-Up Compound;


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         (C)  any Product containing (a) or (b);

         (D)  methods of making any of (a), (b) or (c);

         (E)  any component of (c);

         (F)  any intermediate in the making of any of (a), (b), (c) or (e);

         (G)  any method of using any of (a), (b), (c), (e), or (f); and/or

         (H)  any use of (a), (b) or (c).

    1.38 "PHASE I," "PHASE II," AND "PHASE III" shall mean the phases of the
         clinical development of pharmaceuticals as defined in the United
         States Federal Food, Drug and Cosmetics Act and/or applicable
         regulations promulgated thereunder, as amended from time to time, or
         any comparable foreign regulations.

    1.39 "PRESCRIPTION AUDIT" shall mean an audit conducted pursuant to
         Article 7.7.

    1.40 "PRODUCT" shall mean any pharmaceutical product containing a Licensed
         Compound in any formulation or mode of administration. 

    1.41 "PROMOTION" shall mean, as applied to a Party, the promotion,
         marketing and selling of the Product, including in particular, the
         detailing of the Product to physicians, solely by that Party through
         its sales force in the USA under the Trademark and the Regulatory
         Approval held by Wyeth-Ayerst, and Wyeth-Ayerst's distribution system.

    1.42 "PSYCHIATRIST MARKET" shall mean all psychiatrists prescribing
         pharmaceutical products in the USA during the Term of Co-Promotion.

    1.43 "REGULATORY APPROVAL" shall mean all authorizations by the competent
         authorities which are required for the regular marketing, promotion,
         pricing and sale of the Product in a given country or regulatory
         jurisdiction.

    1.44 "SCHEDULED PRODUCT" shall mean a Product that has received Regulatory
         Approval in the USA as a Schedule I, II, III or IV product, as defined
         in 21 CFR Part 1308.

    1.45 "TERM OF CO-PROMOTION" shall mean the period extending from First
         Commercial Sale in the USA until the later of (i) expiration of the
         last to expire patent within the Patent Rights necessary to make, use,
         import, offer for sale or sell the Product in the USA, or (ii) fifteen
         (15) years from First Commercial Sale in the USA.


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    1.46 "THIRD PARTY(IES)" shall mean any person(s) or entity(ies) other than
         CoCensys, Wyeth-Ayerst or their Affiliates.

    1.47 "TRADEMARK" shall mean the trademark under which the Product shall be
         marketed as set out in Article 9.

    1.48 "USA" shall mean the United States of America, its territories and
         possessions and the Commonwealth of Puerto Rico.

    1.49 "WYETH-AYERST PATENT RIGHTS" shall mean all Patent Rights owned or
         Controlled by Wyeth-Ayerst.  A list of the Wyeth-Ayerst Patent Rights
         existing as of the Effective Date is set out in Exhibit E hereto. 

2.  DEVELOPMENT OF LICENSED COMPOUND.

    2.1  GENERAL OBLIGATIONS.  Wyeth-Ayerst shall be responsible for the
         design, implementation and funding of all Development.  Wyeth-Ayerst
         agrees to use Commercially Reasonable Efforts to conduct the
         Development with the intent of obtaining Regulatory Approval in the
         USA and bringing a Product in the Field to the market as soon as
         reasonably practicable. Wyeth-Ayerst shall ensure that the Development
         is carried out adhering to Wyeth-Ayerst's ethical and safety
         standards.  Wyeth-Ayerst shall have the right to subcontract with
         third parties any of its Development obligations hereunder, without
         the consent or approval of CoCensys.  In the event Wyeth-Ayerst
         intends to exercise its right to subcontract one or more of its
         Development obligations, it shall notify CoCensys of such intent and
         CoCensys shall have the right to submit one or more proposals to
         Wyeth-Ayerst to perform such one or more parts of such Development
         activities.  Wyeth-Ayerst shall consider all such proposals submitted
         by CoCensys on an equal basis with proposals submitted by Third Party
         subcontractors for the same Development tasks.

    2.2  RECORDS, REPORTS AND INFORMATION EXCHANGE.

         2.2.1   TECHNOLOGY AND INFORMATION TRANSFER.  CoCensys will provide to
                 Wyeth-Ayerst all Know-How as Wyeth-Ayerst deems necessary to
                 carry out the Development and [     *     ] of the Product and
                 to obtain Regulatory Approval.  All information transferred,
                 provided or exchanged under this Article 2.2.1 will be subject
                 to the confidentiality requirements set forth in Article 17.

         2.2.2   RECORD KEEPING.  Wyeth-Ayerst will maintain records in
                 sufficient detail and in good scientific manner appropriate
                 for Regulatory Approval and patent purposes.


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         2.2.3   COMMUNICATION REGARDING THE DEVELOPMENT PROGRESS.  A project
                 team shall be appointed by Wyeth-Ayerst and representatives
                 thereof shall meet with representatives of CoCensys on a
                 regular basis (but no less often than every six (6) months) to
                 keep CoCensys apprised of the Development progress and to
                 bring to its attention any problems or issues which may have
                 an impact on the timing of the Development (e.g., regulatory
                 submissions).

    2.3  COMPLIANCE.  Wyeth-Ayerst shall comply with all GLP, GCP and GMP in
         the conduct of the Development.

    2.4  FUNDING OF THE DEVELOPMENT.  Wyeth-Ayerst shall be responsible for one
         hundred percent (100%) of all costs, fees and expenses associated with
         the Development and the obtaining of all Regulatory Approvals.

    2.5  REGULATORY APPROVALS.  [     *     ] shall file all regulatory
         dossiers under its name.  [     *     ] shall own all Regulatory
         Approvals.  [     *     ] shall have the right of reference to the
         extent necessary to exercise its rights or to meet its obligations
         hereunder.  [     *     ] shall be responsible for all communications
         with regulatory agencies, subject to its obligation to keep the Joint
         Marketing Committee informed.


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3.  BACK-UP PROGRAM.

    3.1  PURPOSE.  During the Back-Up Program Term, CoCensys shall use
         Commercially Reasonable Efforts to conduct research, and identify,
         characterize and submit to Wyeth-Ayerst Back-Up Compound Candidates.

    3.2  BACK-UP PROGRAM TERM.  The term of the Back-Up Program shall commence
         on the Effective Date and terminate on the [     *     ] anniversary
         thereof.  Such term may be extended for [          *          ] upon
         120 days prior written notice to [     *     ].  Any Epalons in the
         process of being evaluated for meeting the criteria set forth in
         Exhibit A under the Back-Up Program as of the termination of the
         Back-Up Program shall be submitted to Wyeth-Ayerst promptly thereafter
         to the extent they meet the criteria set forth on Exhibit A.  At the
         end of the Back-Up Program Term, Wyeth-Ayerst shall have rights only
         to those Back-Up Compound Candidates [          *          ] or
         thereafter pursuant to the third sentence of this Article 3.2.

    3.3  REPORTING AND SUBMISSION OF BACK-UP COMPOUND CANDIDATES.  Within
         thirty (30) days of the Effective Date, and at least every six (6)
         months during the Back-Up Program Term, CoCensys shall provide
         Wyeth-Ayerst with a report on the status of the Back-Up Program and
         shall submit on an on-going basis all Back-Up Compound Candidates.

    3.4  BACK-UP COMPOUND DEVELOPMENT.  Wyeth-Ayerst may, at any time and upon
         written notice to CoCensys, (i) elect to [          *          ].

    3.5  BACK-UP PROGRAM FUNDING.  Wyeth-Ayerst shall fund the Back-Up Program
         at the level of Three Million Dollars ($3,000,000) annually, in
         quarterly installments, until the end of the Back-Up Program Term,
         with the first payment of Seven Hundred Fifty Thousand Dollars
         ($750,000) due upon the execution of this Agreement, and the next
         quarter's payment due on the first quarter anniversary thereof. 

    3.6  PARTIAL REIMBURSEMENT OF BACK-UP PROGRAM FEES.  If CO 2-6749 fails to
         meet Wyeth-Ayerst's criteria for [     *     ], and if at such time,
         no Back-Up Compound Candidate has been submitted to Wyeth-Ayerst which
         Wyeth-Ayerst agrees meets the criteria set forth in Exhibit A, then
         Wyeth-Ayerst shall have the right to so notify CoCensys and CoCensys
         will reimburse Wyeth-Ayerst fifty percent (50%) of the funds paid for
         the Back-Up Program to CoCensys as of the date of such notification
         and the Back-Up Program shall immediately terminate.  Such
         reimbursement shall be due within thirty (30) days of receipt of such
         notice, and, at CoCensys' option, shall be in the form either of cash
         or of CoCensys Common Stock.  If reimbursement is to be in CoCensys
         Common Stock,


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         CoCensys will issue to Wyeth-Ayerst that number of shares of CoCensys
         Common Stock which, when multiplied by the Market Price equals the
         amount of such reimbursement.  For purposes of this Article 3.6,
         "Market Price" shall mean the average closing price per share of
         CoCensys Common Stock for twenty (20) trading days prior to the date
         of notification of such reimbursement.

    3.7  APPLICABILITY OF THIS AGREEMENT.  Unless otherwise provided, all of
         the Parties' rights and obligations under this Agreement, including
         those regarding the development, manufacture, distribution, marketing,
         sale, promotion, profit-sharing and royalties of CO 2-6749 and the
         Product are applicable to any Back-Up Compound Candidate either
         replacing CO 2-6749 or otherwise developed in the Field.  The
         provisions herein regarding CO 2-6749 and the Product shall apply to
         such Back-Up Compound Candidate MUTATIS MUTANDIS.

4.  INDICATIONS; EXCLUSIVITY.

    4.1  INDICATIONS.  Wyeth-Ayerst shall have the right to conduct Development
         with respect to any Licensed Compound.  Wyeth-Ayerst [          *      
            ] covenants that it shall not conduct clinical trials of any
         Licensed Compound for [     *     ] in the USA except as provided
         under this Article 4.1.  If Wyeth-Ayerst discovers or determines that
         any Licensed Compound may have efficacy in the treatment of [     *    
         ] and if Wyeth-Ayerst desires to pursue clinical trials of such
         Licensed Compound for [     *     ] it will promptly notify CoCensys
         in writing and disclose to CoCensys its rationale therefor. 
         Wyeth-Ayerst shall have the right to pursue such clinical trials for [ 
            *     ] and market and sell such Licensed Compound as though it
         were a Product developed and sold for [     *     ] under this
         Agreement, subject to the Parties negotiation of terms and conditions,
         including royalty rates, whether and on what terms such Licensed
         Compound will be Co-Promoted by the Parties, and other appropriate
         payment and other terms.  Following such negotiation of such terms and
         conditions the Parties shall either enter into a separate agreement or
         amend this Agreement to so provide for such terms and conditions.  In
         the event the Parties are unable to come to agreement as to the
         appropriate terms and conditions for the development and
         commercialization of such Licensed Compound for [     *     ] by
         Wyeth-Ayerst, the matter shall be referred to the Chief Executive
         Officer of CoCensys and the President of Wyeth-Ayerst Laboratories, an
         Affiliate of Wyeth-Ayerst, for good faith resolution, for a period of
         [     *     ].  If such matter is not resolved by the end of such [    
         *     ] period, the Parties shall be [          *          ]. 
         Notwithstanding the 


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         foregoing, it is expressly understood and agreed that [     *     ] as
         used in this Agreement shall not include [          *          ].

    4.2  NO [     *     ] BY COCENSYS [     *     ].  During the term of this
         Agreement, CoCensys shall not [          *          ].

    4.3  NO [          *          ].  During the term of this Agreement,
         CoCensys will not [          *          ] except under the terms of
         this Agreement, or as otherwise agreed to by Wyeth-Ayerst.

5.  LICENSE GRANTS.

    5.1  LICENSES TO WYETH-AYERST.  Subject to the other provisions of this
         Agreement, CoCensys hereby grants to Wyeth-Ayerst:

         5.1.1   A license in the USA, under its Intellectual Property Rights,
                 to develop, manufacture and have manufactured, import, use,
                 market, offer for sale and sell Products, in the Field, and,
                 subject to Article 4.1, for any New Indication.  Such license
                 shall be exclusive except as to CoCensys, who shall have the
                 right to Promote and Co-Promote the Product in the Field, or
                 outside the Field (where agreed by the Parties pursuant to
                 Article 4.1), in the USA as and to the extent set forth in
                 this Agreement.

         5.1.2   The licenses granted in Article 5.1.1 shall not be
                 sublicensable by Wyeth-Ayerst in the USA without the consent
                 of CoCensys, such consent not to be unreasonably withheld,
                 except to Affiliates of Wyeth-Ayerst, and only for so long as
                 such Affiliates remain Affiliates.

         5.1.3   Unless otherwise provided in this Agreement, Wyeth-Ayerst
                 covenants that it shall not, nor shall it cause any Affiliate
                 to, use or practice directly or indirectly any CoCensys
                 Know-How, and, until expiration thereof, any CoCensys Patent
                 Rights for any purposes other than the development,
                 manufacture, importation, use, marketing, offer for sale or
                 sale of the Product.

    5.2  LICENSES TO COCENSYS.  Subject to the other provisions of this
         Agreement, Wyeth-Ayerst hereby grants to CoCensys:

         5.2.1   A non-exclusive license to practice and use Wyeth-Ayerst's
                 Intellectual Property Rights to the extent needed to Promote
                 and Co-Promote the Product in the Field in the USA under this
                 Agreement and otherwise comply with its obligations under this
                 Agreement;


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         5.2.2   The license granted in Article 5.2.1 shall not be
                 sublicensable by CoCensys without the consent of Wyeth-Ayerst,
                 such consent not to be unreasonably withheld, except to
                 Affiliates of CoCensys, and only for so long as such
                 Affiliates remain Affiliates.

         5.2.3   Unless otherwise provided in this Agreement, CoCensys
                 covenants that it shall not, nor shall it cause any Affiliate
                 to, use or practice directly or indirectly any Wyeth-Ayerst
                 Know-How, and until the expiration thereof, any Wyeth-Ayerst
                 Patent Rights for any purposes other than the marketing,
                 promotion and offering for sale of the Product as provided in
                 this Agreement.

    5.3  LICENSES UPON EXPIRATION.  Upon expiration of the Term of
         Co-Promotion, and provided all sums owing to CoCensys have been paid,
         Wyeth-Ayerst shall have a [          *          ] where agreed under
         Article 4.1, under any remaining CoCensys Intellectual Property
         Rights.

6.  CONSIDERATION TO COCENSYS.

    6.1  UPFRONT FEE.  In consideration for the efforts expended by CoCensys
         prior to the Effective Date and its ongoing assistance with respect to
         the identification and development of CO 2-6749, Wyeth-Ayerst shall
         pay to CoCensys, simultaneous with the execution of this Agreement,
         Five Million Dollars US (US $5,000,000).

    6.2  EQUITY INVESTMENT.  Simultaneous with the execution of this Agreement,
         Wyeth-Ayerst shall enter into a Preferred Stock Purchase Agreement
         with CoCensys, in substantially the form attached hereto as Exhibit F,
         pursuant to which Wyeth-Ayerst shall invest a total amount of Five
         Million Dollars US (US $5,000,000) in convertible preferred capital
         stock of CoCensys.

    6.3  PAYMENTS.  In further consideration of CoCensys' continuing assistance
         in research and development of the Product, Wyeth-Ayerst shall pay to
         CoCensys the following amounts at the time of the following
         achievements with respect to the Product:

         6.3.1   [          *          ]  Upon [          *          ] under
                 this Agreement or Agreement (No. 2), Wyeth-Ayerst shall make
                 to CoCensys a non-refundable payment of [        *        ].

         6.3.2   [          *          ]  Upon [     *     ] Wyeth-Ayerst shall
                 promptly notify CoCensys in writing of such determination and
                 shall make to CoCensys a non-refundable payment of [    *   ].


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         6.3.3   [     *     ].  Upon the [          *          ] Wyeth-Ayerst
                 shall make to CoCensys a non-refundable payment of [         
                 *          ].

         6.3.4   [     *     ].  Upon [     *     ] for the Product,
                 Wyeth-Ayerst shall make to CoCensys a non-refundable payment
                 of [          *          ].

         6.3.5   [     *     ].  Upon [          *          ] Wyeth-Ayerst
                 shall make to CoCensys a non-refundable payment of [     *    
                 ] if the Product is a [     *     ] Product or [     *     ]
                 if the Product is a [     *     ] Product.


    6.4  METHOD OF PAYMENT OF PAYMENTS.  All payments shall be made by
         Wyeth-Ayerst to CoCensys by way of wire transfer to CoCensys within
         thirty (30) days from the date corresponding to the event triggering
         the milestone payment.

    6.5  PAYMENTS FOR BACK-UP COMPOUND CANDIDATES.  Each payment payable
         pursuant to this Article 6 shall [     *     ] be payable [     *    
         ] for the Product [          *          ].

    6.6  ROYALTIES AND PROFIT SHARING.  Wyeth-Ayerst shall, in addition to the
         payments set forth above, also pay to CoCensys royalties and such
         other amounts as set forth in Article 7.

7.  CO-PROMOTION OF THE PRODUCT IN THE USA.

    7.1  PROMOTION AND CO-PROMOTION RIGHTS.

         7.1.1   COCENSYS' RIGHTS TO PROMOTE AND CO-PROMOTE.  With respect to
                 the commercialization of the Product in the USA,
                 notwithstanding the exclusive rights granted to Wyeth-Ayerst,
                 CoCensys shall have:  (i) the right to Promote the Product in
                 the Neurologist Market, and (ii) the right to Co-Promote the
                 Product in the Psychiatrist Market during the Term of
                 Co-Promotion.  The Parties agree and understand that all other
                 rights to Promote, market and otherwise commercialize the
                 Product shall reside with Wyeth-Ayerst, including in
                 particular, the right to Promote the Product in All Other
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    7.1.2     CONDUCT OF PROMOTION AND CO-PROMOTION.  CoCensys and Wyeth-Ayerst
              shall each use Commercially Reasonable Efforts to Promote and
              Co-Promote the Product pursuant to the terms and conditions
              hereof.  The Joint Marketing Committee shall oversee and
              implement all such Promotion and Co-Promotion activities, based
              on the principle of maximizing profits from sales of the Product
              in the USA during the Term of Co-Promotion.  Each Party shall
              cause its sales force, and all other employees and approved
              agents and representatives, to comply with all applicable laws,
              regulations and guidelines in connection with the Co-Promotion of
              the Product, including the Prescription Drug Marketing Act and
              the Federal Anti-Kickback Statute.

    7.1.3     ELECTION OF CO-PROMOTION RIGHT.  CoCensys shall provide
              Wyeth-Ayerst written notice prior to [     *     ] for the
              Product as to whether CoCensys elects to exercise its right to
              Promote and Co-Promote the Product in the USA.  If CoCensys
              elects not to so participate, Wyeth-Ayerst shall have the
              exclusive right to commercialize, market, Promote and sell the
              Product in the USA, subject to the payment to CoCensys of a
              running royalty on Net Sales in the USA at the rates set forth in
              Article 7.1.5.

    7.1.4     TERMINATION OF RIGHTS BY COCENSYS.  Notwithstanding an election
              by CoCensys to Promote and Co-Promote the Product, CoCensys may
              elect to forego its right to Promote and Co-Promote the Product
              if [          *          ] upon [     *     ] months prior
              written notice to Wyeth-Ayerst.

    7.1.5     ROYALTIES IN THE EVENT COCENSYS DOES NOT PROMOTE AND CO-PROMOTE.
              In the event (i) CoCensys elects not to Promote and Co-Promote
              the Product pursuant to Article 7.1.3, (ii) CoCensys elects to
              cease to Promote and Co-Promote the Product pursuant to Article
              7.1.4, or (iii) CoCensys's rights to Promote and Co-Promote the
              Product are terminated pursuant to Article 13.4 or Article
              13.7.1, Wyeth-Ayerst shall have the exclusive right to market,
              sell and Promote the Product in the USA and CoCensys shall
              receive a running royalty on Net Sales in the USA until the end
              of the Term of Co-Promotion as follows:

                   (a)  [     *     ] PRODUCT ROYALTY.  If the Product is a
                        [     *     ] Product, Wyeth-Ayerst shall pay to
                   CoCensys the following running marginal royalty:


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                   (i)       For annual Net Sales of all [     *     ] Products
                             which are less than or equal to [     *     ]
                             million, [     *     ] of such Net Sales per year;
                             and

                   (ii)      For annual Net Sales of all [     *     ] Products
                             which are more than [     *     ] million but less
                             than or equal to [     *     ] million,
                             [     *     ] of such Net Sales per year; and

                   (iii)     For annual Net Sales of all [     *     ] Products
                             which are more than [     *     ] million,
                             [     *     ] of such Net Sales per year.

              (b)  [     *     ] PRODUCT ROYALTY.  If the Product is a
                   [     *     ] Product, Wyeth-Ayerst shall pay to CoCensys
                   the following running marginal royalty:

                   (i)       For annual Net Sales of all [     *     ] Products
                             which are less than or equal to [     *     ]
                             million, [     *     ] of such Net Sales per year;
                             and

                   (ii)      For annual Net Sales of all [     *     ] Products
                             which are more than [     *     ] million but less
                             than or equal to [     *     ] million, [     *
                             ] of such Net Sales per year; and

                   (iii)     For annual Net Sales of all [     *     ] Products
                             which are more than [     *     ] million, [     *
                                ] of such Net Sales per year.

    7.1.6     TERMINATION OF RIGHTS BY WYETH-AYERST.  Wyeth-Ayerst may elect to
              forego its right to Promote and Co-Promote the Product at any
              time following [     *     ] months prior written notice to
              CoCensys, in


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              which case CoCensys shall obtain the exclusive, subject to
              Wyeth-Ayerst's retained right to manufacture Product in the USA
              for sale outside the USA right (with the right to sublicense) to
              manufacture, have manufactured, use, market and sell the Product
              in the USA, and Wyeth-Ayerst shall receive a running royalty on
              Net Sales in the USA until the end of the Term of Co-Promotion as
              follows:

                   (a)  [     *     ] PRODUCT ROYALTY.  If the Product is a
                        [     *     ] Product, CoCensys shall pay to Wyeth-
                        Ayerst the following running marginal royalty:

                        (i)       For annual Net Sales of all [     *     ]
                                  Products which are less than or equal to
                                  [     *     ] million, [     *     ] of such
                                  Net Sales per year; and

                        (ii)      For annual Net Sales of all [     *     ]
                                  Products which are more than [     *     ]
                                  million but less than or equal to
                                  [     *     ] million, [     *     ] of such
                                  Net Sales per year; and

                        (iii)     For annual Net Sales of all [     *     ]
                                  Products which are more than [     *     ]
                                  million, [     *     ] of such Net Sales per
                                  year.

                   (b)  [     *     ] PRODUCT ROYALTY.  If the Product is a
                        [     *     ] Product, CoCensys shall pay to Wyeth-
                        Ayerst the following running marginal royalty:

                        (i)       For annual Net Sales of all [     *     ]
                                  Products which are less than or equal to
                                  [     *     ] million, [     *     ] of such
                                  Net Sales per year; and


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                        (ii)    For annual Net Sales of all [     *     ]
                                Products which are more than [     *     ]
                                million but less than or equal to [     *     ]
                                million, [     *     ] of such Net Sales per
                                year; and

                        (iii)   For annual Net Sales of all [     *     ]
                                Products which are more than [     *     ]
                                million, [     *     ] of such Net Sales per
                                year.

              In the event Wyeth-Ayerst makes the election set forth in this
              Article 7.1.6, Wyeth-Ayerst shall [     *     ].

    7.1.7     NO REINSTATEMENT.  A Party's rights to Promote or Co-Promote the
              Product may not be reinstated after delivery of a notice of early
              termination thereof.

7.2 JOINT MARKETING COMMITTEE.  The Parties agree that the Promotion and
    Co-Promotion will be managed by a Joint Marketing Committee (the "JMC").

    7.2.1     COMPOSITION.  No later than the Beginning of Phase III for the
              Product, each of the Parties will appoint [     *     ]
              representatives to the JMC.  The chairperson of the JMC will be
              [     *     ].  A Party may change any of its representatives at
              any time by giving written notice to the other Party.

    7.2.2     RESPONSIBILITIES.  The JMC will:

              (a)  monitor compliance with the Marketing Plan and approve any
                   immaterial change in the Marketing Plan; and

              (b)  oversee the Promotion and Co-Promotion as discussed in this
                   Article 7, including determining the appropriate level of
                   effort of each Party in the markets in which they Co-Promote
                   in a manner commensurate with each Party's economic interest
                   in such market.

    7.2.3     MEETINGS OF THE JMC.  The chairperson of the JMC shall call
              meetings when deemed appropriate, currently anticipated to be no
              less frequently than once every three (3) months.  If possible,
              the meetings shall be held in person, or where appropriate, by
              video or telephone conference.  The chairperson shall determine
              the form of the meeting.  Additional participants may be invited
              by any member to attend meetings where


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              appropriate (e.g., representatives of regulatory affairs or
              outside consultants).  Such additional participants shall have no
              vote.  Minutes of each meeting of the JMC shall be exchanged for
              review and comment by the members.  Thereafter, they shall be
              signed by the chairperson.

    7.2.4     VOTING OF THE JMC. The JMC shall make decisions by majority vote,
              with at least one consenting vote of each Party's JMC members.
              If the required majority for a decision cannot be found and all
              the members of each Party take the same opposing positions in a
              matter which either Party deems to be of major importance, the
              matter shall be handled pursuant to Article 7.2.6.  Voting by
              proxy is permissible.  Urgent matters (including regulatory and
              adverse event matters) may be decided by unanimous vote of the
              chairperson and a representative designated by CoCensys.

    7.2.5     ROLE OF CHAIRPERSON.  Except as explicitly set forth herein, in
              no event shall the chairperson of the JMC have any additional
              powers or responsibilities beyond those delegated to such person
              by virtue of such person's membership on the JMC.  Without
              limiting the foregoing, it is understood that, except as a voting
              member of the JMC, the chairperson shall not have the power to
              control or dictate decisions or to veto any decisions reached by
              the committee under the decision-making processes set forth in
              Article 7.2.4.

    7.2.6     DISPUTE RESOLUTION.  If the JMC is unable to resolve, after
              thirty (30) days, a dispute regarding any issue presented to it
              or arising in it, such dispute will be referred to the Chief
              Executive Officer of CoCensys and the President of Wyeth-Ayerst
              Laboratories, an Affiliate of Wyeth-Ayerst for good faith
              resolution, for a period of ninety (90) days.  If such dispute is
              not resolved by the end of such ninety (90) day period, the
              Parties shall be free to pursue any legal or equitable remedy
              available to them.

7.3 MARKETING PLAN.  The Promotion and Co-Promotion of the Product will be
    governed by a marketing plan (the "Marketing Plan").  The Marketing Plan
    will describe fully, to the extent practicable, the proposed plan for
    commercialization of the Product in the USA, including overall marketing
    strategy, anticipated marketing, sales and promotion efforts by each Party
    in each of the Neurologist Market, the Psychiatrist Market and All Other
    Markets, market and sales forecasts, pricing analysis and estimated launch
    date, guidelines for discounting the Product, as well as advertising and
    other promotional materials to be used in the Promotion and Co-Promotion.
    The Marketing Plan will be prepared by Wyeth-Ayerst, after consultation
    with representatives of CoCensys, and will take into consideration factors
    such as market conditions, regulatory factors and competition.  The initial
    Marketing Plan shall be prepared under the direction of and adopted by the
    JMC no later than six (6) months after the first filing of the NDA for the
    Product (unless otherwise agreed by the Parties).  Such Marketing


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    Plan shall thereafter be reviewed and, where necessary, updated, at least
    every three (3) months (or more frequently if so determined by the JMC).

7.4 SALES FORCE EXPENSES.  [          *          ].

7.5 DETERMINATION AND ALLOCATION OF GROSS MARGIN AND ROYALTIES.

    7.5.1     PSYCHIATRY MARKET.  In consideration for each Party's
              Co-Promotion efforts in the Psychiatry Market, Wyeth-Ayerst shall
              be entitled to [          *          ] and CoCensys shall be
              entitled to [     *     ] of the Gross Margin from sales of the
              Product in the Psychiatry Market, as determined by a Prescription
              Audit.

    7.5.2     NEUROLOGIST MARKET.  In consideration for CoCensys' Promotion
              efforts in the Neurologist Market, CoCensys shall be entitled to
              [     *     ] and Wyeth-Ayerst shall be entitled to [     *     ]
              of the Gross Margin from sales of the Product in the Neurologist
              Market, as determined by a Prescription Audit.

    7.5.3     ALL OTHER MARKETS.  In consideration for Wyeth-Ayerst's Promotion
              efforts in All Other Markets, Wyeth-Ayerst shall be entitled to
              [     *     ] of the Gross Margin from sales of the Product in
              All Other Markets, as determined by a Prescription Audit, and
              shall pay to CoCensys a running royalty on Net Sales in the USA
              until the end of the Term of Co-Promotion as follows:

                   (a)  [     *     ] PRODUCT ROYALTY.  If the Product is a
                        [     *     ] Product, Wyeth-Ayerst shall pay to
                        CoCensys the following running marginal royalty:

                        (i)     For annual Net Sales of all [     *     ]
                                Products which are less than or equal to
                                [     *     ] million, [     *     ] of such
                                Net Sales per year; and

                        (ii)    For annual Net Sales of all [     *     ]
                                Products which are more than [     *     ]
                                million but less than or equal to [     *     ]
                                million, [     *     ] of such Net Sales per
                                year; and


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                        (iii)   For annual Net Sales of all [     *     ]
                                Products which are more than [     *     ]
                                million, [     *     ] of such Net Sales per
                                year.

                   (b)  [     *     ] PRODUCT ROYALTY.  If the Product is a
                        [     *     ] Product, Wyeth-Ayerst shall pay to
                   CoCensys the following running marginal royalty:

                        (i)     For annual Net Sales of all [     *     ]
                                Products which are less than or equal to
                                [     *     ] million, [     *     ] of such
                                Net Sales per year; and

                        (ii)    For annual Net Sales of all [     *     ]
                                Products which are more than [     *     ]
                                million but less than or equal to [     *     ]
                                million, [     *     ] of such Net Sales per
                                year; and

                        (iii)   For annual Net Sales of all [     *     ]
                                Products which are more than [     *     ]
                                million, [     *     ] of such Net Sales per
                                year.

7.6 ADJUSTMENT TO ROYALTIES.

    7.6.1     GENERIC PRODUCTS.  In the event that, during the Term of
              Co-Promotion, a generic version (i.e., same chemical entity) of
              the Product is introduced in the USA by a Third Party, and if
              unit sales for all such generic product(s) constitute more than
              [     *     ] of the combined unit sales in the USA of both the
              Product and any such generic product(s), then the annual royalty
              amount owed by one Party under Article 7.1.5, 7.1.6 or 7.5.3
              shall be reduced by [     *     ] such reduction to be done in
              the fourth quarter payment as described in Article 7.8.

    7.6.2     [     *     ]  In the event one Party (the "Royalty Paying
              Party") owes royalties to the other Party (the "USA Royalty
              Receiving Party") pursuant to Article 7.1.5, 7.1.6 or 7.5.3
              hereunder with respect to Net Sales in the USA and, at the end of
              a given calendar year [     *     ] (where [


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                    *     ] = [          *          ]= [          *          ];
              and [     *     ]= [     *     ] then the Royalty Paying Party
              will provide written notice to the USA Royalty Receiving Party of
              such fact and the Parties shall meet promptly thereafter to
              discuss and develop in good faith a commercially reasonable plan
              to [          *          ].  In the event that, at any time after
              the eight (8) month anniversary of the date of the first meeting
              to discuss the [     *     ] (where [     *     ] =
              [          *          ] then, for so long as [     *     ], an
              amount equal to [     *     ] may be deducted from the royalty
              due the USA Royalty Receiving Party for such year; PROVIDED,
              HOWEVER, in no event shall the royalty due the USA Royalty
              Receiving Party be less than that paid at the lowest applicable
              rate (i.e., [     *     ] for [     *     ] Products and
              [     *     ] for [     *     ] Products, or [     *     ]
              respectively, where Article 7.6.1 applies) set forth in Article
              7.1.5, 7.1.6 or 7.5.3.

    7.7   PRESCRIPTION AUDIT.  Wyeth-Ayerst shall retain, at its own cost, a
          Third Party such as IMS America, Ltd. to conduct an audit of the
          prescribing activity in all markets and to provide the prescription
          data necessary to calculate Net Sales in each of the Neurologist
          Market, the Psychiatry Market and All Other Markets (the
          "Prescription Audit").  Wyeth-Ayerst will provide to CoCensys such
          data consistent with its obligations to IMS and such data will be
          the property of Wyeth-Ayerst.

    7.8   PAYMENT AND REPORTING.  Within three (3) months after the close of
          each calendar quarter, or earlier if possible, during the Term of
          Co-Promotion (i.e., on or before the last day of each of the months
          of June, September, December and March), Wyeth-Ayerst shall furnish
          to CoCensys a statement (the "P&L Statement") setting forth Net
          Sales in the USA and all data on which the determination of Gross
          Margin was calculated.  Wyeth-Ayerst will submit any amount due to
          CoCensys pursuant to Articles 7.1, 7.5 and 7.6 with the P&L
          Statement.  For any given royalty period during the first three
          quarters of the year, Wyeth-Ayerst shall pay to CoCensys the royalty
          at the lowest rate specified in Articles 7.1.5 or 7.5.3 applicable
          to the then current year-to-date Net Sales level in the USA.  Each
          statement provided at the end of the fourth quarter shall contain a
          reconciliation of actual royalty payments made during that year and
          the amount actually owed for such year, and any amounts owing to
          CoCensys shall be paid to CoCensys at the time of such fourth
          quarter statement, in accordance with the terms of this Article 7.8.
          If the Term of Co-Promotion ends during an accounting quarter, the
          amounts due hereunder shall be calculated for such shortened
          calendar quarter.  Any adjustments to royalties owed in the USA


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          pursuant to Articles 7.6.1 and 7.6.2 shall be done at the end of the
          calendar year in which such event giving rise to such adjustment
          occurred; PROVIDED, HOWEVER, that following the entry of one or more
          generic products in any year, and where royalties were reduced for
          that year pursuant to Article 7.6.1, the following year's royalty
          rate for quarterly royalty payments shall be at [     *     ] with a
          reconciliation upward at year end in the event sales of such generic
          product(s) do not rise to the level specified in Article 7.6.1.

    7.9   PROMOTIONAL AND ADVERTISING MATERIALS.  The Parties shall
          disseminate in the USA only those promotional and advertising
          materials which have been provided or approved for use by the Joint
          Marketing Committee, the cost of which shall be the responsibility
          of Wyeth-Ayerst.  All such materials shall be consistent with the
          relevant Marketing Plan approved by the Joint Marketing Committee
          and neither Party shall make any claims or representations in
          respect of the Product that have not been approved by the Joint
          Marketing Committee.  All such written and visual materials and all
          documentary information, promotional material, and oral
          presentations (where practical) regarding the promotion of the
          Product will state this arrangement and will display the
          Wyeth-Ayerst and CoCensys names and logos with equal prominence, as
          permitted by applicable law.

    7.10  PRICING.  The Parties will discuss, and the Marketing Plan will
          include, the general operating guidelines and strategies for the
          pricing and discounting of the Product in the USA; PROVIDED,
          HOWEVER, that [          *          ] as to pricing and discounting
          in the USA.

    7.11  NO DELEGATION.  Each of the Parties may use only its own employees
          or the employees of one or more of its Affiliates in the course of
          exercising its Promotion and Co-Promotion rights under this
          Agreement, unless an alternative arrangement is approved in advance
          by the JMC.  Notwithstanding the foregoing, in the event CoCensys is
          exclusively marketing, selling, or promoting the Product in the USA,
          it shall be free to delegate or sublicense such rights to any Third
          Party.

    7.12  RETURNS.  Wyeth-Ayerst shall be responsible for handling all returns
          relating to Product.  Any Product returned to CoCensys shall be
          shipped by CoCensys to the address designated by Wyeth-Ayerst.

    7.13  ORDERS.  All customer orders for the Product shall be received and
          executed by Wyeth-Ayerst.

    7.14  SAMPLES.  Each of the Parties will keep accurate records as to the
          distribution of samples of Products and comply with all applicable
          laws, rules and regulations dealing with the distribution of
          samples.


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    7.15  COMPLETION OF SALES.  All sales of the Product will be completed,
          distributed, accounted for, billed and booked by Wyeth-Ayerst.

    7.16  TRAINING.  Consistent with the Marketing Plan, but not less than
          ninety (90) days prior to the commencement of the Term of
          Co-Promotion for the Product, Wyeth-Ayerst and CoCensys shall
          conduct joint initial training of each Party's sales force at a site
          to be determined by the JMC.  [          *          ]

    7.17  EXCHANGE OF MARKETING INFORMATION.  From time-to-time the Joint
          Marketing Committee will develop call lists, schedules, and other
          appropriate information for the purpose of determining the
          physicians and other persons involved in the drug purchase
          decision-making process to whom CoCensys and Wyeth-Ayerst,
          respectively, may detail and otherwise promote each Product.  The
          Parties agree to cooperate in finding an inexpensive and expeditious
          way to provide a call list and other information indicating the
          identity of those physicians and other persons involved in the
          decision-making process regarding the purchase of pharmaceuticals.

8.  ACCOUNTS AND RECORDS; WITHHOLDING TAX.

    8.1   RECORDS.  Wyeth-Ayerst shall keep accurate books and accounts of
          record in connection with the manufacture, use and/or sale by or for
          it of the Products in sufficient detail to permit accurate
          determination of all figures necessary for verification of
          royalties, profits, milestone payments and other compensation
          required to be paid hereunder.  Wyeth-Ayerst shall maintain such
          records for a period of three (3) years after the end of the year in
          which they were generated.

    8.2   AUDITS.  CoCensys, through an independent certified public
          accountant reasonably acceptable to Wyeth-Ayerst, shall have the
          right, at its own expense, to access the books and records of
          Wyeth-Ayerst for the sole purpose of verifying statements furnished
          by Wyeth-Ayerst pursuant to Article 7.8.  Such access shall be
          conducted after reasonable prior written notice to Wyeth-Ayerst and
          during ordinary business hours and shall not be more frequent than
          once during each calendar year.  CoCensys agrees to keep in strict
          confidence all information learned in the course of such audit,
          except when it is necessary to reveal such information in order to
          enforce its rights under this Agreement.  CoCensys' right to have
          such records examined shall survive termination or expiration of
          this Agreement.  In the event such audit reveals an underpayment of
          [     *     ] or more of the amount actually due, Wyeth-Ayerst shall
          reimburse CoCensys for the costs of such audit in addition to
          promptly remitting to CoCensys the amount of any underpayment.

    8.3   SALES BY SUBLICENSEES.  In the event Wyeth-Ayerst grants licenses or
          sublicenses to others to make or sell the Product, such licenses or
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         an obligation for the licensee or sublicensee to account for and
         report its Net Sales of such Products on the same basis as if such
         sales were Net Sales by Wyeth-Ayerst, and CoCensys shall receive
         royalties in the same amounts as if the Net Sales of the licensee or
         sublicensee were Net Sales of Wyeth-Ayerst.

    8.4  WITHHOLDING.  All taxes, assessments and fees of any nature levied or
         incurred on account of any payments accruing under this Agreement, by
         national, state or local governments, will be assumed and paid by
         Wyeth-Ayerst, except taxes levied thereon as income to CoCensys and if
         such taxes are required to be withheld by Wyeth-Ayerst they will be
         deducted from payments due to CoCensys and will be timely paid by
         Wyeth-Ayerst to the proper taxing authority for the account of
         CoCensys, a receipt or other proof of payment therefor secured and
         sent to CoCensys as soon as practicable.

9.  TRADEMARKS.

    [     *     ] shall select and own the Trademarks for marketing the Product
    in the USA.  [     *     ] for (i) registration of such Trademarks and
    (ii) bringing, maintaining and prosecuting any action to protect or defend
    such Trademarks shall be borne [          *          ]  At the termination
    of this Agreement, [     *     ] shall continue to have unrestricted
    ownership of such Trademark(s) in the USA.  [     *     ] rights in this
    Article are subject to Article 7.1.6.

10. MANUFACTURING AND DISTRIBUTION.

    10.1 PRIMARY MANUFACTURE.  [     *     ] shall manufacture or have
         manufactured its requirements for clinical and commercial supplies of
         the Product.  In fulfilling its manufacturing obligations hereunder, 
         [     *     ] will use at least the same level of effort as it employs
         for its other products of similar scientific and commercial promise. 
         If [     *     ] elects to have the Product manufactured, 
         [     *     ] shall favorably consider using, but shall not be 
         obligated to use, [          *          ] as its manufacturing 
         sublicensee.

    10.2 SECOND SOURCE.  In order to ensure an uninterrupted supply of the
         Product, the Parties intend to identify a Third Party manufacturer
         (the "Second Source") to manufacture the Product in the event that 
         [     *     ] is unable to supply all of the reasonably anticipated
         requirements of the Product.  If the Second Source manufactures the
         Product pursuant to this Article 10.2, Wyeth-Ayerst and CoCensys shall
         cooperate and assist each other to obtain, transfer or use any
         licenses, registrations or information reasonably required to permit
         such Second Source to manufacture the Product.


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    10.3 EXCHANGE OF INFORMATION.  Subject to Article 17, the Parties undertake
         to exchange and to use diligent efforts to cause Third Party
         manufacturers to exchange, on a regular basis, all data and know-how
         relating to the manufacture of the Product.

    10.4 COMPLIANCE.  Any manufacture of the Product for sale in the USA shall
         be performed in full compliance with United States GMP and all
         applicable laws and regulations.  The JMC shall be entitled to audit
         such compliance and in particular the quality assurance program for
         the manufacture of the Product.

11. PROSECUTION, MAINTENANCE AND INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS.

    11.1 PATENTABLE INVENTIONS.

         11.1.1    Wyeth-Ayerst shall own all Inventions made solely by
                   itsemployees and agents, and all patent applications and
                   patents claiming such Inventions.  CoCensys shall own all
                   Inventions made solely by its employees and agents, and all
                   patent applications and patents claiming such Inventions. 
                   All Inventions made jointly by employees or agents of
                   CoCensys and employees or agents of Wyeth-Ayerst and all
                   patent applications and patents claiming such Inventions
                   shall be owned jointly by CoCensys and Wyeth-Ayerst.  All
                   determinations of inventorship under this Article 11.1.1
                   shall be in accordance with U.S. law.

         11.1.2    Wyeth-Ayerst and CoCensys shall each disclose to the other
                   and discuss any Inventions and the desirability of filing a
                   United States patent application covering the Invention, as
                   well as any foreign counterparts.  The Party owning the
                   Invention shall make the final decision with respect to any
                   such filings.  With respect to jointly owned Inventions, the
                   Parties shall determine which Party shall file and prosecute
                   any patent applications thereon.  [     *     ] shall be
                   responsible for expenses for preparing and prosecuting joint
                   patent applications in the USA.

         11.1.3    Each Party shall have the right to select patent counsel and
                   to take such other actions as are reasonably appropriate to
                   prepare, file, prosecute and maintain patent protection with
                   respect to its Inventions arising under this Article 11.1.

    11.2 PROSECUTION AND MAINTENANCE OF PATENT RIGHTS.  In the USA, each Party
         shall be responsible for prosecuting and maintaining its own Patent
         Rights, subject to Article 11.1.2.  To facilitate such
         decision-making, each Party will appoint a "patent coordinator", who
         will have the authority to make such decision on behalf 


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         of such Party.  All expenses for filing, prosecuting and maintaining
         the CoCensys Patent Rights in the USA shall be borne [     *     ]

    11.3 PATENT EXTENSIONS.  The Party holding a Patent Right, if requested by
         and with the assistance of the other Party, shall apply in a timely
         manner for such patent term extensions or patent restoration
         certificates for such Patent Right as are available under the Federal
         Drug Price Competition and Patent Term Restoration Act of 1984 (Pub.
         L. No. 98-417), and any amendments thereof or any successor acts
         thereto in the USA.  [     *     ] expenses incurred in connection
         with such patent term extensions or patent restoration certificates
         shall be borne [          *          ].

    11.4 COOPERATION.  Each of the Parties shall execute or have executed by
         its appropriate employees, representatives, agents, and contractors
         such documents as may be necessary to obtain, perfect or maintain any
         Patent Rights filed or to be filed pursuant to this Agreement, and to
         cooperate with the other Party so far as reasonably necessary with
         respect to furnishing all information and data in its possession
         reasonably necessary to obtain or maintain such Patent Rights.

    11.5 INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS.

         11.5.1

              (a)  If either Party should become aware of any infringement or
                   threatened infringement or misappropriation, as the case may
                   be, in the USA of any Intellectual Property Rights of the
                   other Party, it shall promptly notify such other Party in
                   writing.  As soon as practicable the Parties shall confer on
                   the particulars of such infringement or misappropriation and
                   the possible courses of action to be taken.  The Party
                   holding the affected Intellectual Property Rights shall have
                   the right, but not the obligation, to institute, prosecute
                   and control any legal proceedings in its own name and by its
                   own counsel and at its own expense, subject to
                   Article 11.5.1(d), to prevent or restrain such infringement,
                   and the other Party shall have the right, [     *     ] to
                   be represented in such action by its own counsel.  If one
                   Party brings any such action or proceeding, the other Party
                   hereby consents to being joined as a party plaintiff where
                   necessary and, in case of joining, such other Party agrees
                   to give the first Party reasonable assistance and authority
                   to file and to prosecute such suit, at the exercise of the
                   Party bringing such suit.


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              (b)  Notwithstanding the foregoing, the Parties shall jointly
                   determine which Party shall have the primary right and
                   responsibility (but not the obligation) to institute,
                   prosecute, and control any action or proceeding with respect
                   to infringement or misappropriation of jointly owned
                   Intellectual Property Rights in the USA and the other Party
                   shall have the right, [     *     ] to be represented by its
                   counsel.  Each Party hereby consents to the filing of any
                   such action by the other Party with respect to any jointly
                   owned Patent Rights in accordance with this Article
                   11.5.1(b).

              (c)  If one Party alone prosecutes an infringement or
                   misappropriation of Intellectual Property Rights, 
                   [         *         ] any damages and costs recovered in 
                   any proceedings or by way of settlement under Articles 
                   11.5.1(a) and 11.5.1(b) above or Article 11.5.2 shall 
                   [         *         ] as applicable.

              (d)  If both Parties participate in prosecuting an infringement
                   or misappropriation of Intellectual Property Rights, the
                   actual costs and expenses of all suits brought by either
                   Party under this Article 11.5.1 shall be [        *        ] 
                   Any remaining damages shall then be split [     *     ] to
                   CoCensys and [     *     ] to Wyeth-Ayerst.

         11.5.2    If the Party having the primary right to institute,
                   prosecute, and control such infringement or misappropriation
                   action under Article 11.5.1 fails to do so within a period
                   of one hundred twenty (120) days after receiving notice of
                   the infringement, or if that Party, after initiating an
                   action, determines to discontinue such action, the other
                   Party shall have the right to bring and control or take over
                   any such action by counsel of its own choice, and at its own
                   expense, subject to Article 11.5.1(c) unless prevented from
                   doing so by the laws of the country where the infringement
                   or misappropriation occurred or is threatened.

         11.5.3    In connection with any proposed settlement in respect of any
                   infringement or threatened infringement of any Intellectual
                   Property Rights, the Party intending to settle shall notify
                   and consult with the other Party as to the terms of
                   settlement, whose written consent shall be required prior to
                   any such settlement, such consent shall not be unreasonably
                   withheld.

         11.5.4    In connection with any action taken by either Party against
                   a Third Party to protect or enforce any Intellectual
                   Property Rights, the 


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                   other Party shall, if requested, consult with the Party
                   taking such action, and make available as witnesses its
                   employees or as evidence any materials, and/or data as are
                   reasonably necessary for the furtherance of such action. 
                   The expenses in connection with the providing of witnesses
                   and/or the making available of any materials and/or data
                   shall be [          *          ]

11.6     INFRINGEMENT OF THIRD PARTY PATENT RIGHTS.

         11.6.1    If Wyeth-Ayerst should be of the opinion that it cannot
                   make, import, use, market and/or sell the Product in the USA
                   under its own Intellectual Property Rights or those licensed
                   to it by CoCensys under this Agreement without infringing a
                   Third Party's patent, it shall notify CoCensys.  Both
                   Parties then shall seek an opinion of patent counsel
                   acceptable to both Parties.  If such patent counsel concurs
                   with Wyeth-Ayerst's opinion, they shall jointly or
                   independently endeavor to secure a license from the Third
                   Party on terms that are acceptable to both Parties.

         11.6.2    If, in the opinion of patent counsel selected under
                   Article 11.6.1, the Third Party patent, if litigated, would
                   be found invalid or not be infringed by the manufacture or
                   sale of the Product or if the Parties otherwise mutually
                   agree to obtain a license to such Third Party patent, the
                   Parties shall proceed in accordance with the terms of this
                   Agreement, unless an action for infringement is brought
                   against one or both Parties.

         11.6.3    If either Party is sued for patent infringement of any Third
                   Party patents arising out of the manufacture, use, sale or
                   importation of the Product in the USA, the Parties shall
                   promptly meet to discuss the course of action to be taken to
                   resolve or defend any such infringement litigation.  Each
                   Party shall provide the other with such assistance as is
                   reasonably necessary and shall cooperate in the defense of
                   any such action.  [     *     ] of any cost/expense of
                   defending such action incurred by Wyeth-Ayerst in the USA
                   and any damages and/or other compensation imposed on
                   Wyeth/Ayerst in such country may be deducted by Wyeth-Ayerst
                   from any amounts otherwise due CoCensys under this Agreement
                   in the form of royalties and/or profit-sharing for the USA,
                   PROVIDED, HOWEVER, in no event shall royalty and/or profit
                   sharing amounts due CoCensys for such country be reduced by
                   more than [     *     ] in any given calendar year as a
                   result of this sentence.


                                         28.

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<PAGE>

12. FORCE MAJEURE.

    Neither Party shall be liable to the other for delay or failure in the
    performance of the obligations on its part contained in this Agreement if
    and to the extent that such failure or delay is due to circumstances beyond
    its control which it could not have avoided by the exercise of reasonable
    diligence.  It shall notify the other Party promptly should such
    circumstances arise, giving an indication of the likely extent and duration
    thereof, and shall use all commercially reasonable efforts to resume
    performance of its obligations as soon as practicable.

13. TERM AND TERMINATION.

    13.1 GENERAL CONDITIONS OF EXPIRATION AND TERMINATION.

         13.1.1    Any permitted sublicenses granted hereunder shall
                   automatically terminate or expire at the same time as this
                   Agreement expires (insofar as they haven't already
                   terminated), except where, and to the extent, any license
                   granted hereunder survives expiration of this Agreement, as
                   expressly provided in this Agreement.  Upon early
                   termination of this Agreement for any reason, if any
                   permitted sublicensee is not then in default under its
                   sublicense agreement with Wyeth-Ayerst, then such
                   sublicensee shall automatically have a license under this
                   Agreement as a direct licensee of CoCensys, on economic
                   terms as are set forth herein with respect to Wyeth-Ayerst
                   and otherwise with the same rights and obligations as
                   Wyeth-Ayerst under this Agreement.

         13.1.2    The provisions of Articles 8, 9, 11, 14, 15, and 17 shall
                   survive termination or expiration of this Agreement.

         13.1.3    Termination or expiration of this Agreement shall not
                   operate to deprive either Party of any rights or remedies
                   either at law or in equity or to relieve either Party of any
                   of its obligations incurred prior to the effective date of
                   such termination or expiration.

    13.2 TERM.  Unless earlier terminated as set out in this Agreement, the
         term of this Agreement shall end when all the respective royalty and
         profit payment obligations of the Parties under this Agreement have
         expired.

    13.3 TERMINATION FOR BREACH.

         13.3.1    TERMINATION FOR BREACH.  Except as otherwise provided in
                   this Section 13.3.1, either Party may terminate this
                   Agreement for material breach by the other Party, which
                   breach remains uncured for [     *     ] in the case of
                   nonpayment of any amount due 


                                         29.

* Confidential treatment requested

<PAGE>

                   and [     *     ] for all other breaches, each measured from
                   the date written notice of such breach is given to the
                   breaching party, or, if such breach is not susceptible of
                   cure within such [     *     ] period and the breaching
                   party uses diligent good faith efforts to cure such breach,
                   for [     *     ] after written notice to the breaching
                   party.

         13.3.2    BREACH BY WYETH-AYERST.  If termination is due to a material 
                   breach by Wyeth-Ayerst, all rights granted to Wyeth-Ayerst
                   under this Agreement shall revert to CoCensys, provided that
                   [          *          ] as of the date of such termination.

    13.4 BREACH BY COCENSYS.  In the event of an uncured material breach by
         CoCensys, [          *          ] subject to the payment of royalties
         as provided in Article 7.1.5.

    13.5 NO LIMIT ON REMEDIES.  Nothing herein shall exclude or limit any
         remedies or entitlements whatsoever which the law confers to either
         Party in the event of a breach of contractual obligations by the other
         Party.

    13.6 UNILATERAL TERMINATION BY WYETH-AYERST.  Wyeth-Ayerst shall have the
         right to unilaterally terminate this Agreement upon [     *     ]
         written notice.  In the event of such termination by Wyeth-Ayerst, all
         Wyeth-Ayerst's worldwide rights to the Licensed Compound and the
         Product shall revert to CoCensys.  Wyeth-Ayerst shall not be obligated
         to further fund the Development or the Back-Up Program or to make any
         other payments under this Agreement for events which occur after the
         effective date of such termination.  After the termination date,
         Wyeth-Ayerst will make its relevant personnel, relevant data and other
         resources available as are reasonably necessary to effect an orderly
         transition of Development and commercialization of the Product for a
         period of [     *     ] after termination.  In the event of such
         termination, Wyeth-Ayerst shall (i) [          *          ] (ii)
         transfer to CoCensys [          *          ] and (iii) 
         [          *           ]

    13.7 DETERMINATION OF CO-PROMOTION RIGHTS UPON CHANGE IN CONTROL.

         13.7.1    In the event of a Change of Control (as defined below) of 
                   [     *     ] promptly shall notify [     *     ] of such 
                   Change in Control and [     *     ] shall have the right, 
                   for a period of [


                                         30.

* Confidential treatment requested

<PAGE>

                        *     ] from receipt of such notice, and upon 
                   [     *     ] written notice to [     *     ] (the "Notice 
                   of Intent"), to [          *          ]

         13.7.2    For purposes of this Article 13.7, "Change in Control" shall
                   mean (1) a merger or consolidation in which a Party hereto
                   is not the surviving corporation; (2) a reverse merger in
                   which a Party hereto is the surviving corporation but the
                   shares of such Party's voting stock outstanding immediately
                   preceding the merger are converted by virtue of the merger
                   into other property, whether in the form of securities, cash
                   or otherwise; or (3) if, after giving effect to any
                   agreements among stockholders of a Party hereto, any person
                   holds and may vote in excess of 50% of such Party's voting
                   stock.

         13.7.3    A Change of Control of [     *     ] shall have no effect on
                   this Agreement.

14. ASSIGNMENT.

    14.1 ASSIGNMENT TO AFFILIATES.  Either Party may assign any of its rights
         or obligations under this Agreement in any country to any Affiliates,
         for so long as they remain Affiliates; provided, however, that such
         assignment shall not relieve the assigning Party of its
         responsibilities for performance of its obligations under this
         Agreement.

    14.2 OTHER PERMITTED ASSIGNMENT.  Either Party may assign its rights or
         obligations under this Agreement in connection with a merger or
         similar reorganization or the sale of all or substantially all of its
         assets, subject to provisions of Article 13.7, [          *          ]
         provided, that in the event of such merger, reorganization or sale, 
         [          *          ]  All other assignments by any Party shall 
         [          *          ]

    14.3 BINDING NATURE OF ASSIGNMENT.  This Agreement shall be binding upon
         and inure to the benefit of the successors and permitted assigns of
         the Parties.  Any assignment not in accordance with this Article 14
         shall be void.


                                         31.

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<PAGE>

15. INDEMNIFICATION.

    15.1 CROSS INDEMNIFICATION.  

         15.1.     Each Party hereby agrees to save, defend and hold the other
                   Party and its agents and employees harmless from and against
                   any and all suits, claims, actions, demands, liabilities,
                   expenses and/or losses, including reasonable legal expense
                   and attorneys' fees, brought by a Third Party or that arise
                   in connection with any claim brought by a Third Party
                   ("Losses") resulting directly from the manufacture, use,
                   handling, storage, sale or other disposition of Products in
                   the USA to the extent such Losses result solely from (i) the
                   negligence of the indemnifying party or breach by the
                   indemnifying party of any provision of this Agreement, (ii)
                   failure of the indemnifying party to manufacture or have
                   manufactured Products (bulk or finished  form) according to
                   cGMP or Product specifications, or (iii) marketing
                   activities of the indemnifying party contrary to applicable
                   governmental regulations or outside the approved labeling of
                   the Product.

         15.1.2    In the event CoCensys is seeking indemnification under
                   Article 15.1.1, it shall inform Wyeth-Ayerst of a claim as
                   soon as is reasonably practicable after it receives notice
                   of the claim, shall permit Wyeth-Ayerst to assume direction
                   and control of the defense of the claim (including the right
                   to settle the claim solely for monetary consideration), and
                   shall cooperate as requested (at the expense of
                   Wyeth-Ayerst) in the defense of the claim.

         15.1.3    In the event Wyeth-Ayerst is seeking indemnification under   
                   Article 15.1.1, it shall inform CoCensys of a claim as soon
                   as is reasonably practicable after it receives notice of the
                   claim, shall permit CoCensys to assume direction and control
                   of the defense of the claim (including the right to settle
                   the claim solely for monetary consideration), and shall
                   cooperate as requested (at the expense of CoCensys) in the
                   defense of the claim.

    15.2 INSURANCE.  Each Party further agrees to use reasonable commercial
         efforts to obtain and maintain, during the term of this Agreement,
         Comprehensive General Liability Insurance, including Products
         Liability, with reputable and financially secure insurance carriers to
         cover its indemnification obligations under Articles 15.1 and 15.2 or
         self-insurance, with limits of not less than $5,000,000 per occurrence
         and in the aggregate.


                                         32.
<PAGE>

16. WARRANTIES AND REPRESENTATIONS.

    16.1 GENERAL.  Each Party hereby warrants to the other:

         16.1.1    that it has full power and authority to execute and deliver
                   this Agreement and to perform the obligations on its part
                   hereunder; and

         16.1.2    that the execution and delivery by it of this Agreement and
                   the performance of its obligations hereunder have been duly
                   approved by all necessary corporate action and do not
                   require any shareholder action or approval.

         16.1.3    that to the best of its knowledge, the manufacture, use,     
                   importation, offer for sale or sale of CO 2-6749 will not
                   infringe any Third Party patent in the USA.

    16.2 USC/ROCKEFELLER LICENSE.  Wyeth-Ayerst acknowledges and understands
         that certain technologies sublicensed to Wyeth-Ayerst hereunder were
         licensed to CoCensys under terms of that certain Exclusive License
         Agreement among CoCensys, The University of Southern California and
         the Rockefeller University dated August 28, 1990 (the "USC/Rockefeller
         License"), a copy of which has been provided to Wyeth-Ayerst. 
         CoCensys has received a letter from the University of Southern
         California, dated April 11, 1997, permitting CoCensys to grant to
         Wyeth-Ayerst a sublicense under such license.

17. CONFIDENTIAL INFORMATION.

    17.1 INFORMATION.  Each Party shall keep all information received from the
         other Party (the "Information") confidential and shall not disclose
         nor use the Information without the other Party's written consent
         except to the extent contemplated by this Agreement.  This restriction
         shall not, however, prevent disclosure of the Information if and to
         the extent that disclosure is required by law, PROVIDED THAT the
         disclosing Party informs the other Party without delay of any such
         requirement, in order to allow such other Party to object to such
         disclosure and to seek an appropriate protective order or similar
         protection prior to disclosure.

    17.2 EXCEPTIONS.  The above obligations shall not apply or shall cease to
         apply to Information which:

         17.2.1    is now, or hereafter becomes, through no act or failure to
                   act on the part of the receiving Party, generally known or
                   available;

         17.2.2    is known by the receiving Party at the time of receiving
                   such information, as evidenced by its written records;


                                         33.
<PAGE>

         17.2.3    is hereafter furnished to the receiving Party by a Third
                   Party, as a matter of right and without restriction on
                   disclosure;

         17.2.4    is independently developed by or for the receiving Party
                   without any breach of this Article 17; or

         17.2.5    is the subject of a written permission to disclose provided
                   by the disclosing Party.

    17.3 PERMITTED DISCLOSURES.  Information may be disclosed to employees,
         agents, consultants, sublicensees or suppliers of the recipient Party
         or its Affiliates, but only to the extent required to accomplish the
         purposes of this Agreement and only if the recipient Party obtains
         prior agreement from its employees, agents, consultants, sublicensees
         or suppliers to whom disclosure is to be made to hold in confidence
         and not make use of such information for any purpose other than those
         permitted by this Agreement.  Each Party will use at least the same
         standard of care as it uses to protect proprietary or confidential
         information of its own to ensure that such employees, agents,
         consultants, sublicensees or suppliers do not disclose or make any
         unauthorized use of the Information.

    17.4 DISCLOSURE OF AGREEMENT.  Neither CoCensys nor Wyeth-Ayerst shall
         release to any Third Party or publish in any way any non-public
         information with respect to the terms of this Agreement or concerning
         their cooperation without the prior written consent of the other,
         which consent will not be unreasonably withheld or delayed; provided;
         however that either Party may disclose the terms of this Agreement to
         the extent required to comply with applicable laws, including without
         limitation the rules and regulations promulgated by the Securities and
         Exchange Commission and the Party intending to disclose the terms of
         this Agreement shall provide the nondisclosing party an opportunity to
         review and comment on the intended disclosure which is reasonable
         under the circumstances.  Notwithstanding any other provision of this
         Agreement, each Party may disclose the terms of this Agreement to
         lenders, investment bankers and other financial institutions of its
         choice solely for purposes of financing the business operations of
         such Party either (i) upon the written consent of the other Party or
         (ii) if the disclosing Party uses reasonable efforts to obtain a
         signed confidentiality agreement with such financial institution with
         respect to such information, upon terms substantially similar to those
         contained in this Article 17.

    17.5 PUBLICITY.  Subject to Article 17.4, all publicity, press releases and
         other announcements relating to this Agreement or the transaction
         contemplated hereby shall be reviewed in advance by, and shall be
         subject to the approval of, both Parties.

    17.6 PUBLICATION.  The Parties shall cooperate in appropriate publication
         of the results of research and development work performed pursuant to
         this Agreement, but


                                         34.
<PAGE>

         subject to their predominating interest in obtaining patent protection
         for any patentable subject matter.  The determination of authorship
         for any paper shall be in accordance with accepted scientific
         practice.  Notwithstanding anything in this Article 17.6 to the
         contrary, all publication and presentations of the results of research
         and development work performed pursuant to this Agreement must be
         approved in advance by both Parties.

18. MISCELLANEOUS.

    18.1 NO WAIVER OF CONTRACTUAL RIGHTS.  The failure of either Party to
         require performance by the other Party of any of that other Party's
         obligations hereunder shall in no manner affect the right of such
         Party to enforce the same at a later time.  No waiver by any Party
         hereto of any condition, or of the breach of any provision, term,
         representation or warranty contained in this Agreement, whether by
         conduct or otherwise, in any one or more instances, shall be deemed to
         be or construed as a further or continuing waiver of any such
         condition or breach, or of any other condition or of the breach of any
         other provision, term representation or warranty hereof.

    18.2 EXECUTION AND AMENDMENTS.

         18.2.1    Each Party shall execute and deliver all such instruments
                   and perform all such other acts as the other Party may
                   reasonably request in order to carry out the transactions
                   contemplated by this Agreement.

         18.2.2    This Agreement may not be amended or modified except by
                   written instrument signed by or on behalf of both Parties.

    18.3 SEVERABILITY.  If a court or other tribunal of competent jurisdiction
         should hold any term or provision of this agreement to be excessive,
         or invalid, void or unenforceable, the offending term or provision
         shall be deleted, and, if possible, replaced by a term or provision
         which, so far as practicable achieves the legitimate aims of the
         Parties.

    18.4 RELATIONSHIP BETWEEN THE PARTIES.  Both Parties are independent
         contractors under this Agreement.  Nothing contained in this Agreement
         is intended nor shall be construed so as to constitute CoCensys or
         Wyeth-Ayerst as partners or joint venturers with respect to this
         Agreement.  Neither Party shall have the express or the implied right
         nor authority to assume or create any obligations on behalf of or in
         the name of the other Party, nor to bind the other Party to any other
         contract, agreement or undertaking with any Third Party.

    18.5 CORRESPONDENCE AND NOTICES.


                                         35.
<PAGE>

         18.5.1    Correspondence, reports, documentation, and any other
                   communication in writing between the Parties in the course
                   of ordinary implementation of this Agreement shall be
                   delivered by hand, sent by facsimile, or by airmail to any
                   one member of the JMC appointed by the Party which is to
                   receive such written communication, or any other way as the
                   JMC deems appropriate.

         18.5.2    Extraordinary notices and communications (including but not
                   limited to notices of termination, force majeure, material
                   breach, change of address) shall be in writing and sent by
                   prepaid registered or certified air mail, or by facsimile
                   confirmed by prepaid registered or certified air mail
                   letter, and shall be deemed to have been properly served to
                   the addressee upon receipt of such written communication.

         18.5.3    In the case of CoCensys, the proper address for
                   communications and for all payments shall be:

                   CoCensys, Inc.
                   213 Technology Drive
                   Irvine, California 92618, USA
                   Attn:  Chief Executive Officer

                   and in the case of Wyeth-Ayerst, the proper address for
                   communications and for all payments shall be:

                   Wyeth-Ayerst Laboratories
                   555 Lancaster Avenue
                   St. Davids, PA  19087
                   Attn:  Senior Vice President, Global Business Development

         With a copy to:

                   American Home Products Corporation
                   5 Giralda Farms
                   Madison, NJ  07940
                   Attn:  Senior Vice President and General Counsel

    18.6 CHOICE OF LAW.  This Agreement is subject to and governed by the laws
         of the State of Delaware, excluding its conflict of laws provisions.

    18.7 COUNTERPARTS.  This Agreement may be executed in one or more
         counterparts, each of which shall be an original and all of which
         shall constitute together the same document.


                                         36.
<PAGE>

This Agreement together with its Exhibits and further agreements mentioned
herein and Agreement (No. 2) constitutes the entire agreement of the Parties
with respect to the subject matter hereof as of its date, and supersedes all
prior agreements, understandings, representations and proposals, written or
oral, relating thereto.

AMERICAN HOME PRODUCTS CORPORATION     COCENSYS, INC.



-------------------------------        -------------------------------
Name                                   Name
Title                                  Title


                                         37.
<PAGE>

                                      EXHIBIT A

                         BACK-UP COMPOUND CANDIDATE CRITERIA
             -----------------------------------------------------------
             -----------------------------------------------------------

              CRITERION FOR IND-TRACKING            ACTIVITIES REQUIRED
             -----------------------------------------------------------
             -----------------------------------------------------------
                               [          *          ]


* Confidential treatment requested

<PAGE>

                                      EXHIBIT B



                               [          *          ]


* Confidential treatment requested

<PAGE>

                                      EXHIBIT C

                                COCENSYS PATENT RIGHTS



US  ISSUE DATE     EXPIRATION DATE     CORRESPONDING FOREIGN APPLICATION
-------------------------------------------------------------------------

                               [          *          ]


* Confidential treatment requested

<PAGE>

                                      EXHIBIT D


                          DEFINITION OF FULLY BURDENED COST

The following expenses are manufacturing expenses which are prepared in
accordance with generally accepted accounting principles consistently applied.

THE FOLLOWING EXPENSES ARE INCLUDED IN MANUFACTURING COSTS:


                               [          *          ]


* Confidential treatment requested

<PAGE>

                                      EXHIBIT E

                             WYETH-AYERST PATENT RIGHTS


                               [          *          ]


* Confidential treatment requested

<PAGE>

                                      EXHIBIT F

                               STOCK PURCHASE AGREEMENT
<PAGE>

                                 COCENSYS, INC.

                                   -----------


                       PREFERRED STOCK PURCHASE AGREEMENT

                                   -----------


                                  MAY   , 1997

<PAGE>

                                TABLE OF CONTENTS
                                                                        PAGE

1.   PURCHASE AND SALE . . . . . . . . . . . . . . . . . . . . . . .      1
     1.1  Shares.. . . . . . . . . . . . . . . . . . . . . . . . . .      1
     1.2  Closing Date . . . . . . . . . . . . . . . . . . . . . . .      1
     1.3  Delivery . . . . . . . . . . . . . . . . . . . . . . . . .      1

2.   REPRESENTATIONS AND WARRANTIES OF THE COMPANY . . . . . . . . .      1
     2.1  Organization and Standing; Articles and Bylaws . . . . . .      1
     2.2  Authorization. . . . . . . . . . . . . . . . . . . . . . .      1
     2.3  Validity of Shares and Conversion Shares . . . . . . . . .      2
     2.4  Offering . . . . . . . . . . . . . . . . . . . . . . . . .      2
     2.5  Full Disclosure. . . . . . . . . . . . . . . . . . . . . .      2
     2.6  SEC Filings. . . . . . . . . . . . . . . . . . . . . . . .      3
     2.7  Authorized Capital; Outstanding Shares . . . . . . . . . .      3
     2.8  Litigation . . . . . . . . . . . . . . . . . . . . . . . .      3
     2.9  Voting Arrangements. . . . . . . . . . . . . . . . . . . .      3
     2.10 No Conflict; No Violation. . . . . . . . . . . . . . . . .      3
     2.11 Consents and Approvals.. . . . . . . . . . . . . . . . . .      3
     2.12 Absence of Certain Developments. . . . . . . . . . . . . .      4

3.   REPRESENTATIONS AND WARRANTIES OF THE PURCHASER . . . . . . . .      4
     3.1  Legal Power. . . . . . . . . . . . . . . . . . . . . . . .      4
     3.2  Due Execution. . . . . . . . . . . . . . . . . . . . . . .      4
     3.3  Investment Representations . . . . . . . . . . . . . . . .      4

4.   REGISTRATION RIGHTS . . . . . . . . . . . . . . . . . . . . . .      6
     4.1  Definitions. . . . . . . . . . . . . . . . . . . . . . . .      6
     4.2  Registration . . . . . . . . . . . . . . . . . . . . . . .      6
     4.3  Expenses of Registration . . . . . . . . . . . . . . . . .      7
     4.4  Obligations of the Company . . . . . . . . . . . . . . . .      8
     4.5  Indemnification. . . . . . . . . . . . . . . . . . . . . .      9
     4.6  Information by Holder. . . . . . . . . . . . . . . . . . .     11
     4.7  Transfer of Registration Rights. . . . . . . . . . . . . .     11
     4.8  Delay of Registration. . . . . . . . . . . . . . . . . . .     11
     4.9  Rule 144 Reporting.. . . . . . . . . . . . . . . . . . . .     11
     4.10 "Market Stand-Off" Agreement.. . . . . . . . . . . . . . .     12

5.   CONDITIONS TO CLOSING . . . . . . . . . . . . . . . . . . . . .     12
     5.1  Conditions to Obligations of Purchaser . . . . . . . . . .     12
     5.2  Conditions to Obligations of the Company . . . . . . . . .     13

6.   MISCELLANEOUS . . . . . . . . . . . . . . . . . . . . . . . . .     14
     6.1  Governing Law. . . . . . . . . . . . . . . . . . . . . . .     14
     6.2  Survival . . . . . . . . . . . . . . . . . . . . . . . . .     14
     6.3  Successors and Assigns . . . . . . . . . . . . . . . . . .     14


                                        i
<PAGE>

     6.4  Entire Agreement . . . . . . . . . . . . . . . . . . . . .     14
     6.5  Separability . . . . . . . . . . . . . . . . . . . . . . .     14
     6.6  Amendment and Waiver . . . . . . . . . . . . . . . . . . .     14
     6.7  Delays or Omissions. . . . . . . . . . . . . . . . . . . .     15
     6.8  Notices, etc . . . . . . . . . . . . . . . . . . . . . . .     15
     6.9  Finder's Fees. . . . . . . . . . . . . . . . . . . . . . .     16
     6.10 Fees and Expenses. . . . . . . . . . . . . . . . . . . . .     16
     6.11 Information Confidential . . . . . . . . . . . . . . . . .     16
     6.12 Titles and Subtitles . . . . . . . . . . . . . . . . . . .     17
     6.13 Counterparts . . . . . . . . . . . . . . . . . . . . . . .     17


                                       ii
<PAGE>

                                 COCENSYS, INC.

                       PREFERRED STOCK PURCHASE AGREEMENT


     THIS AGREEMENT is made as of May 12, 1997, by and among COCENSYS, INC., a
Delaware corporation (the "Company"), and AMERICAN HOME PRODUCTS CORPORATION, a
Delaware corporation ("Purchaser").

     1.   PURCHASE AND SALE

          1.1  SHARES.  Subject to the terms and conditions hereof, and in
reliance upon the representations, warranties and agreements contained herein,
the Company hereby agrees to issue and sell to Purchaser, and Purchaser hereby
agrees to purchase from the Company, for $5,000,000 (the "Purchase Price"),
100,000 shares (the "Shares") of the Company's Series C Convertible Preferred
Stock (the "Preferred Stock").  The terms of the Preferred Stock are set forth
in the Certificate of Designation annexed hereto as Exhibit A (the "Certificate
of Designation").

          1.2  CLOSING DATE.  The closing of the sale and purchase of the Shares
(the "Closing") shall take place on May 12, 1997 (the "Closing Date").

          1.3  DELIVERY.  At the Closing, the Company will deliver to Purchaser
a certificate or certificates, in such denominations and registered in such
names as Purchaser may designate by notice to the Company, representing the
Shares to be purchased by Purchaser from the Company, dated the Closing Date,
against payment of the Purchase Price by wire transfer, a check made payable to
the order of the Company, or any combination thereof.

     2.   REPRESENTATIONS AND WARRANTIES OF THE COMPANY.

          Except as otherwise specifically disclosed to Purchaser in writing on
the date hereof, the Company hereby represents and warrants to Purchaser as
follows:

          2.1  ORGANIZATION AND STANDING; ARTICLES AND BYLAWS.  The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of Delaware, and has full and requisite power and authority to own
and operate its properties and assets and to carry on its business as presently
conducted and as proposed to be conducted.  The Company is duly qualified as a
foreign corporation to do business in each jurisdiction in which the ownership
of its property or the conduct of its business requires such qualification,
except where the failure to so qualify would not materially or adversely affect
the Company, its business, assets, prospects, condition (financial or otherwise)
or operations.

          2.2  AUTHORIZATION.  The Company has all requisite corporate power and
authority to execute and deliver this Agreement and to perform its obligations
hereunder and to issue the Shares as contemplated herein.  All corporate action
on the part of the Company, its


                                        1
<PAGE>

officers, directors and stockholders necessary for the authorization, execution
and delivery of this Agreement, the performance of all the Company's obligations
hereunder, for the authorization, issuance, sale and delivery of the Shares and
for the reservation for issuance of the shares of Common Stock issuable upon
conversion of the Shares (the "Conversion Shares") has been taken or will be
taken prior to the Closing.  This Agreement, when executed and delivered, shall
constitute a valid and legally binding obligation of the Company in accordance
with its terms, subject to laws of general application relating to bankruptcy,
insolvency and the relief of debtors.

          2.3  VALIDITY OF SHARES AND CONVERSION SHARES.  The sale of the Shares
is not and will not be subject to any preemptive rights or rights of first
refusal that have not been waived and, when issued, sold and delivered in
compliance with the provisions of this Agreement and, as applicable, the
Certificate of Designation, the Shares and the Conversion Shares will be validly
issued, fully paid and nonassessable and will be free of any liens, claims or
encumbrances; PROVIDED, HOWEVER, that the Shares and the Conversion Shares will
be subject to restrictions on transfer under state and/or federal securities
laws, and the Shares will be subject to additional restrictions on transfer, in
each case as set forth herein, or as otherwise required by such laws at the time
a transfer is proposed.

          2.4  OFFERING.  Assuming the accuracy of the representations and
warranties of Purchaser contained in Section 3.3 hereof on the date hereof and
on the Closing Date, the offer, issue, and sale of the Shares, and the issuance
of the Conversion Shares (assuming no change in applicable law on each
Conversion Date), are and will be exempt from the registration and prospectus
delivery requirements of the Securities Act of 1933, as amended (the "1933 Act")
and have been or will be registered or qualified (or are or will be exempt from
registration and qualification) under the registration, permit, or qualification
requirements of all applicable state securities laws.

          2.5  FULL DISCLOSURE.

               (a) The Company has furnished to Purchaser the Company's annual
report on Form 10-K for the fiscal year ended December 31, 1996, as amended (the
"1996 10-K") and the Company's Current Report on Form 8-K dated April 29, 1997
(the "8-K") (the "SEC Documents").  The Company warrants that, as of their
respective dates (or if amended, as of the date of such amendment), the SEC
documents complied as to form with the requirements of the Securities Exchange
Act of 1934 (the "1934 Act"), and the information contained in such documents
did not contain any untrue statement of a material fact, and did not omit to
state any material fact necessary to make any statement, in light of the
circumstances under which such statement was made, not misleading.  The Company
further warrants that the 1996 10-K, as modified by the 8-K, does not contain
any untrue statement of a material fact and does not omit to state any material
fact necessary to make the statements therein not misleading as of the date
hereof.

               (b)  The Company has not filed with the Securities and Exchange
Commission (the "SEC") any reports under the 1934 Act since the date of the 8-K.


                                        2
<PAGE>

          2.6  SEC FILINGS.  The Company has timely filed with the SEC all
reports and other documents required to be so filed.  The Company agrees that,
as long as the Development and Commercialization Agreement between the Company
and Wyeth-Ayerst Laboratories, dated as of May 12, 1997, is in effect, it will,
upon the Purchaser's written request, promptly furnish to Purchaser all SEC
filings and any annual and quarterly reports furnished to the Company's
stockholders.

          2.7  AUTHORIZED CAPITAL; OUTSTANDING SHARES.  The Company is
authorized to issue 75,000,000 shares of Common Stock and 5,000,000 shares of
Preferred Stock.  As of March 31, 1997, there were 22,516,863 shares of Common
Stock and 100,000 shares of Preferred Stock outstanding.  No shares of capital
stock, or securities convertible into or exercisable for capital stock or other
rights affecting the capital stock, have been issued since such date except for
issuances pursuant to the Company's equity compensation plans or pursuant to
outstanding options, warrants or rights, in each case as described in the 1996
10-K.  All outstanding shares of Common Stock have been duly authorized and
validly issued and are fully paid and non-assessable; and none of the
outstanding shares of Common Stock were issued in violation of the preemptive
rights, if any, of any stockholders of the Company.

          2.8  LITIGATION.  There is no action, suit or proceeding pending, or,
to the Company's knowledge, threatened, against the Company (a) which questions
the validity of this Agreement or the ability of the Company to consummate the
transactions contemplated hereby or (b) which, singly or in the aggregate, if
the subject of unfavorable decision, ruling or finding, would materially
adversely affect the business, properties, prospects, operations, or financial
condition of the Company, in each case as described in the 1996 10-K.

          2.9  VOTING ARRANGEMENTS.  To the best of the Company's knowledge,
there are no outstanding stockholder agreements, voting trusts, proxies or other
arrangements or understandings among the stockholders of the Company relating to
the voting of their respective shares.

          2.10 NO CONFLICT; NO VIOLATION.  The execution, delivery and
performance of this Agreement and consummation of the transactions contemplated
hereby will not (a) violate or conflict with any provisions of the Amended and
Restated Certificate of Incorporation, as amended, or Bylaws of the Company; (b)
result in any breach, conflict with, violation of or default or loss of a
benefit under, or permit the acceleration of any obligation under (in each case,
upon the giving of notice, the passage of time, or both) any mortgage,
indenture, lease, loan agreement or other agreement or instrument, permit,
franchise, license, judgment, order, decree, law, ordinance, rule or regulation
applicable to the Company or its properties.

          2.11 CONSENTS AND APPROVALS.  All consents, approvals, orders, or
authorizations of, or registrations, qualifications, designations, declarations,
or filings with, any federal, state or local governmental authority, required on
the part of the Company in connection with the valid execution, delivery and
performance of this Agreement, the offer, sale or issuance of the Shares, or the
consummation of any other transaction contemplated hereby have been obtained, or
will be effective at the Closing, except for notices required or permitted to be
filed


                                        3
<PAGE>

with certain state and federal securities commissions after the Closing, which
notices will be filed on a timely basis.

          2.12 ABSENCE OF CERTAIN DEVELOPMENTS.  Except as disclosed in or
contemplated by the SEC Documents, since December 31, 1996, the Company has not
(a) incurred or become subject to any material liabilities (absolute or
contingent) except current liabilities incurred, and liabilities under contracts
entered into, in the ordinary course of business, consistent with past
practices; (b) mortgaged, pledged or subjected to lien, charge or any other
encumbrance any of its assets, tangible or intangible; (c) sold, assigned or
transferred any of its assets or canceled any debts or obligations except in the
ordinary course of business, consistent with past practices; (d) suffered any
extraordinary losses, or waived any rights of substantial value; (e) sold,
assigned or transferred to a third party that is not an affiliate (within the
meaning set forth in Rule 405 under the Securities Act of 1933, as amended) any
material patents, trademarks, copyrights, trade secrets or other intangible
assets for compensation less than the fair value of such assets; (f) declared,
paid or otherwise made any dividend or distribution of any kind on its capital
stock; (g) entered into any material transaction other than in the ordinary
course of business, consistent with past practices; or (h) otherwise had any
material change in its condition, financial or otherwise, except for changes in
the ordinary course of business, consistent with past practices, none of which
individually or in the aggregate has been materially adverse.

          2.13 NASDAQ NATIONAL MARKET DESIGNATION.  The Common Stock of the
Company is listed on the Nasdaq National Market and the Company knows of no
reason or set of facts which is likely to result in the delisting of the Common
Stock by the Nasdaq National Market or the inability of such stock to continue
to be included in the Nasdaq National Market.

     3.   REPRESENTATIONS AND WARRANTIES OF THE PURCHASER.

          Purchaser hereby represents and warrants to the Company as follows:

          3.1  LEGAL POWER.  It has the requisite legal power to enter into this
Agreement, to purchase the Shares hereunder, and to carry out and perform its
obligations under the terms of this Agreement.

          3.2  DUE EXECUTION.  This Agreement has been duly authorized, executed
and delivered by it, and, upon due execution and delivery by the Company, this
Agreement will be a valid and binding agreement of it in accordance with its
terms, subject to laws of general application relating to bankruptcy, insolvency
and the relief of debtors.

          3.3  INVESTMENT REPRESENTATIONS.

               (a)  It is acquiring the Shares, and intends to acquire the
Conversion Shares, for its own account, not as nominee or agent, for investment
and not with a view to, or for resale in connection with, any distribution or
public offering thereof within the meaning of the 1933 Act.


                                        4
<PAGE>

               (b)  It understands and agrees that  the Shares have not been
and, when issued, the Conversion Shares will not be, registered under the 1933
Act by reason of a specific exemption therefrom, that they must be held by it
indefinitely, and that it must, therefore, bear the economic risk of such
investment indefinitely, unless a subsequent disposition thereof is registered
under the 1933 Act or is exempt from such registration;  each certificate
representing the Shares and the Conversion Shares will be endorsed with the
following legend:

          THE SECURITIES EVIDENCED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
     UNDER THE 1933 ACT, AND MAY NOT BE SOLD, TRANSFERRED, ASSIGNED OR
     HYPOTHECATED UNLESS (A) PURSUANT TO RULE 144 UNDER THE 1933 ACT OR (B)
     THERE IS AN EFFECTIVE REGISTRATION STATEMENT UNDER THE 1933 ACT COVERING
     SUCH SECURITIES OR (C) THE COMPANY RECEIVES AN OPINION OF COUNSEL FOR THE
     HOLDER OF THESE SECURITIES REASONABLY SATISFACTORY TO THE COMPANY, STATING
     THAT SUCH SALE, TRANSFER, ASSIGNMENT OR HYPOTHECATION IS EXEMPT FROM THE
     REGISTRATION AND PROSPECTUS DELIVERY REQUIREMENTS OF THE 1933 ACT;

(iii) each certificate representing the Shares also will be endorsed with the
following legend:

          THE SECURITIES EVIDENCED BY THIS CERTIFICATE MAY NOT BE SOLD,
     TRANSFERRED, ASSIGNED OR HYPOTHECATED UNLESS (A) THE TRANSFEREE IS AN
     AFFILIATE OF THE HOLDER WITHIN THE MEANING OF RULE 144 UNDER THE 1933 ACT
     OR (B) ALL SUCH SECURITIES ARE TRANSFERRED TO A SINGLE INDIVIDUAL OR
     ENTITY;

and  the Company will instruct any transfer agent not to register the transfer
of any of the Shares unless the conditions specified in the foregoing legends
are satisfied, PROVIDED HOWEVER, that to the extent the legend set forth in
clause (b)(ii) above is no longer required, the Company shall cause its transfer
agent to issue a new certificate or certificates free of such legend.

               (c)  It has been furnished with such materials and has been given
access to such information relating to the Company as it or its qualified
representative has requested and it has been afforded the opportunity to ask
questions regarding the Company and the Shares, all as it has found necessary to
make an informed investment decision.

               (d)  It is an "accredited investor" within the meaning of
Regulation D under the 1933 Act.

               (e)  It was not formed for the specific purpose of acquiring the
Shares or the Conversion Shares.


                                        5
<PAGE>

     4.   REGISTRATION RIGHTS.

          The Company hereby grants to Purchaser the registration rights set
forth in this Section 4, with respect to the Registrable Securities (as
hereinafter defined) owned by Purchaser.

          4.1  DEFINITIONS.  As used in this Section 4:

               (a)  The term "Holder" or "Holders" shall mean (i) Purchaser and
(ii) any other person holding or having the right to acquire Registrable
Securities to whom these registration rights have been transferred pursuant to
Subsection 4.7 hereof.

               (b)  The terms "register," "registered," and "registration" refer
to a registration effected by filing with the SEC a registration statement (the
"Registration Statement") in compliance with the 1933 Act and the declaration or
ordering by the SEC of the effectiveness of such Registration Statement.

               (c)  The term "Registrable Securities" means (i) the shares of
Common Stock of the Company issued upon conversion of the Shares (the
"Conversion Shares") in accordance with the Certificate of Designation and such
other shares of Common Stock of the Company held by Purchaser from time to time
and (ii) any Common Stock of the Company issued as (or issuable upon the
conversion or exercise of any warrant, right, or other security that is issued
as) a dividend or other distribution with respect to, or in exchange or in
replacement of, the shares referred to in clause (i) of this subsection (c);
PROVIDED, HOWEVER, that Registrable Securities shall cease to be Registrable
Securities upon the expiration of the Market Stand-Off Agreement set forth in
Section 4.10 hereof.  In the event of any recapitalization by the Company,
whether by stock split, reverse stock split, stock dividend or the like, the
number of shares of Registrable Securities shall be proportionately increased or
decreased.

          4.2  REGISTRATION.


               (a)  REGISTRATION.  If at any time or from time to time the
Company shall determine to register any of its securities for its own account
(other than a registration relating solely to employee benefit plans or a
registration on Form S-4 relating solely to an SEC Rule 145 transaction) or for
the account of security holders pursuant to demand registration rights, the
Company will:

                    (i)  promptly give to each Holder written notice thereof
(which shall include a list of the jurisdictions in which the Company intends to
attempt to qualify such securities under the applicable blue sky or other state
securities laws); and

                    (ii) include in such registration (and any related
qualification under blue sky laws or other compliance), and in any underwriting
involved therein, all the Registrable Securities specified in a written request
or requests, made within 20 calendar days


                                        6
<PAGE>

after receipt of such written notice from the Company, by any Holder or Holders,
except as set forth in Subsection 4.2(b) below.

               (b)  UNDERWRITING.  If the registration of which the Company
gives notice is for a registered public offering involving an underwriting, the
Company shall so advise the Holders as a part of the written notice given
pursuant to Subsection 4.2(a)(i).  In such event the right of any Holder to
registration pursuant to this Section 4 shall be conditioned upon such Holder's
participation in such underwriting and the inclusion of such Holder's
Registrable Securities in the underwriting to the extent provided herein.  All
Holders proposing to distribute their securities through such underwriting
shall, together with the Company and any other parties distributing their
securities through such underwriting, enter into an underwriting agreement in
customary form with the underwriter or underwriters selected for such
underwriting by the Company.  Notwithstanding any other provision of this
Subsection 4.2, if the underwriter determines that marketing factors require a
limitation of the number of shares to be underwritten, the underwriter may limit
the number of Registrable Securities to be included in the registration and
underwriting, or may exclude Registrable Securities entirely from such
registration and underwriting subject to the terms of this paragraph.  The
Company shall so advise all holders of the Company's securities that would
otherwise be registered and underwritten pursuant hereto, and the number of
shares of such securities, including Registrable Securities, that may be
included in the registration and underwriting shall be allocated in the
following manner:  shares, other than Registrable Securities and other
securities carrying registration rights, requested to be included in such
registration by stockholders shall be excluded and if a limitation on the number
of shares is still required, the number of securities that may be included shall
be allocated among the Holders and holders of securities having PARI PASSU
registration rights, if any, in proportion, as nearly as possible, to the
respective amounts of such securities held by each such holder, in each case at
the time of filing the Registration Statement.  In the event of any underwriter
cutback, if any selling stockholder which is a Holder of Registrable Securities
is a partnership or corporation, the partners, retired partners and stockholders
of such Holder, or the estates and family members of any such partners and
retired partners and any trusts for the benefit of any of the foregoing persons
shall be deemed to be a single "selling Holder", and any pro rata reduction with
respect to such "selling Holder" shall be based upon the aggregate amount of
shares carrying registration rights owned by all entities and individuals
included in such "selling Holder", as defined in this sentence.  No securities
excluded from the underwriting by reason of the underwriter's marketing
limitation shall be included in such registration.  If any Holder disapproves of
the terms of the underwriting, it may elect to withdraw therefrom by written
notice to the Company and the underwriter.  The Registrable Securities so
withdrawn shall also be withdrawn from registration.

          4.3  EXPENSES OF REGISTRATION.  All expenses incurred in connection
with a registration effected pursuant to Subsection 4.2, including without
limitation all registration, filing, and qualification fees (including blue sky
fees and expenses), printing expenses, escrow fees, fees and disbursements of
counsel for the Company and, if there are more than two (2) participating
Holders, of one special counsel for the participating Holders, and expenses of
any special audits incidental to or required by such registration (collectively,
"Registration Expenses"), shall be borne by the Company; provided, however, that
the term Registration


                                        7
<PAGE>

Expenses shall not include, and in no event will the Company be obligated to
pay, stock transfer taxes or underwriters' discounts or commissions relating to
Registrable Securities.

          4.4  OBLIGATIONS OF THE COMPANY.  Whenever required under Section 4.2
to effect the registration of any Registrable Securities, the Company shall, as
expeditiously as reasonably possible:

               (a)  Prepare and file with the SEC a registration statement with
respect to such Registrable Securities and use its best efforts to cause such
registration statement to become effective, and, upon the request of the Holders
of a majority of the Registrable Securities registered thereunder, keep such
registration statement effective until the earlier of (i) one hundred eighty
(180) days or (ii) until the Holder or Holders have completed the distribution
relating thereto; PROVIDED HOWEVER, that the Company may delay the filing of
such registration statement for up to 60 days following such request by giving
notice to Purchaser if the Company shall have determined that the Company may be
required to disclose any material corporate development which disclosure may
have a material effect on the Company.  Following the effectiveness of a
registration statement filed pursuant to Rule 415 under the 1933 Act, the
Company may, at any time, but not more than once in any six-month period,
suspend the effectiveness of such registration statement for up to 60 days, as
appropriate (a "Suspension Period"), by giving notice to Purchaser, if the
Company shall have determined that the Company may be required to disclose any
material corporate development which disclosure may have a material effect on
the Company.  The duration of any Suspension Period shall be added to the period
of time the Company agrees to keep the registration statement effective.
Purchaser agrees that, upon receipt of any notice from the Company of a
Suspension Period, Purchaser shall forthwith discontinue disposition of shares
covered by such registration statement or prospectus until Purchaser (i) is
advised in writing by the Company that the use of the applicable  prospectus may
be resumed, (ii) has received copies of a supplemental or amended prospectus, if
applicable, and (iii) has received copies of any additional or supplemental
filings which are incorporated or deemed to be incorporated by reference in such
prospectus.

               (b)  Prepare and file with the SEC such amendments and
supplements to such registration statement and the prospectus used in connection
with such registration statement as may be necessary to comply with the
provisions of the 1933 Act with respect to the disposition of all securities
covered by such registration statement.

               (c)  Furnish to the Holders such numbers of copies of a
prospectus, including a preliminary prospectus, in conformity with the
requirements of the 1933 Act, and such other documents as they may reasonably
request in order to facilitate the disposition of Registrable Securities owned
by them.

               (d)  Use its best efforts to register and qualify the securities
covered by such registration statement under such other securities or Blue Sky
laws of such jurisdictions as shall be reasonably requested by the Holders,
provided that the Company shall not be required in connection therewith or as a
condition thereto to qualify to do business or to file a general consent to
service of process in any such states or jurisdictions.


                                        8
<PAGE>

               (e)  In the event of any underwritten public offering, enter into
and perform its obligations under an underwriting agreement, in usual and
customary form, with the managing underwriter of such offering and, if requested
by such underwriter, cause appropriate officers to participate in a "road show"
to market such offering.  Each Holder participating in such underwriting shall
also enter into and perform its obligations under such an agreement.

               (f)  Notify each Holder of Registrable Securities covered by such
registration statement, at any time when a prospectus relating thereto is
required to be delivered under the 1933 Act, of the happening of any event as a
result of which the prospectus included in such registration statement, as then
in effect, includes an untrue statement of a material fact or omits to state a
material fact required to be stated therein or necessary to make the statements
therein not misleading in the light of the circumstances then existing.

               (g)  Furnish, at the request of any Holder requesting
registration of Registrable Securities pursuant to this Section 4, on the date
that such Registrable Securities are delivered to the underwriters for sale in
connection with a registration pursuant to this Section 4, if such securities
are being sold through underwriters, on the date that the registration statement
with respect to such securities becomes effective, (i) an opinion, dated such
date, of the counsel representing the Company for the purposes of such
registration, in form and substance as is customarily given to underwriters in
an underwritten public offering, addressed to the underwriters, if any, and to
the Holders requesting registration of Registrable Securities and (ii) a letter
dated such date, from the independent accountants of the Company, in form and
substance as is customarily given by independent accountants to underwriters in
an underwritten public offering, addressed to the underwriters, if any, and to
the Holders requesting registration of Registrable Securities.

          4.5  INDEMNIFICATION.

               (a)  The Company will, and does hereby undertake to, indemnify
and hold harmless each Holder of Registrable Securities, each of such Holder's
officers, directors, partners and agents, and each person controlling such
Holder, with respect to any registration, qualification, or compliance effected
pursuant to this Section 4, and each underwriter, if any, and each person who
controls any underwriter, of the Registrable Securities held by or issuable to
such Holder, against all claims, losses, damages, and liabilities (or actions in
respect thereto) to which they may become subject under the 1933 Act, the 1934
Act, or other applicable law arising out of or based on (i) any untrue statement
(or alleged untrue statement) of a material fact contained in any prospectus,
offering circular, or other similar document (including any related Registration
Statement, notification, or the like) incident to any such registration,
qualification, or compliance, or based on any omission (or alleged omission) to
state therein a material fact required to be stated therein or necessary to make
the statements therein not misleading, or (ii) any violation or alleged
violation by the Company of any law, rule or regulation applicable to the
Company in connection with any such registration, qualification, or compliance,
and will reimburse, as incurred, each such Holder, each such underwriter, and
each such director, officer, partner, agent and controlling person, for any
legal and any other expenses reasonably incurred in connection with
investigating or defending any such claim, loss,


                                        9
<PAGE>

damage, liability, or action; provided that the Company will not be liable in
any such case to the extent that any such claim, loss, damage, liability or
expense, arises out of or is based on any untrue statement or omission based
upon written information furnished to the Company by an instrument duly executed
by such Holder or underwriter and stated to be specifically for use therein.

               (b)  Each Holder will, if Registrable Securities held by or
issuable to such Holder are included in such registration, qualification, or
compliance, indemnify the Company, each of its directors, and each officer who
signs a Registration Statement in connection therewith, and each person
controlling the Company, each underwriter, if any, and each person who controls
any underwriter, of the Company's securities covered by such a Registration
Statement, and each other Holder, each of such other Holder's officers,
partners, directors and agents and each person controlling such other Holder,
against all claims, losses, damages, and liabilities (or actions in respect
thereof) arising out of or based on any untrue statement (or alleged untrue
statement) of a material fact contained in any such Registration Statement,
prospectus, offering circular, or other document, or any omission (or alleged
omission) to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading, and will reimburse, as
incurred, the Company, each such underwriter, each such other Holder, and each
such director, officer, partner, and controlling person, for any legal or any
other expenses reasonably incurred in connection with investigating or defending
any such claim, loss, damage, liability, or action, in each case to the extent,
but only to the extent, that such untrue statement (or alleged untrue statement)
or omission (or alleged omission) was made in such Registration Statement,
prospectus, offering circular, or other document, in reliance upon and in
conformity with written information furnished to the Company by an instrument
duly executed by such Holder and stated to be specifically for use therein.  In
no event will any Holder be required to enter into any agreement or undertaking
in connection with any registration under this Section 4 providing for any
indemnification or contribution obligations on the part of such Holder greater
than such Holder's obligations under this Subsection 4.5.  The liability of each
Holder for indemnification under this Section 4.5 shall not exceed the proceeds
to such Holder from the sale of Registrable Securities.

               (c)  Each party entitled to indemnification under this
Subsection 4.5 (the "Indemnified Party") shall give notice to the party required
to provide such indemnification (the "Indemnifying Party") of any claim as to
which indemnification may be sought promptly after such Indemnified Party has
actual knowledge thereof, and shall permit the Indemnifying Party to assume the
defense of any such claim or any litigation resulting therefrom; provided that
counsel for the Indemnifying Party, who shall conduct the defense of such claim
or litigation, shall be subject to approval by the Indemnified Party (whose
approval shall not be reasonably withheld) and the Indemnified Party may
participate in such defense at the Indemnifying Party's expense if
representation of such Indemnified Party would be inappropriate due to actual or
potential differing interests between such indemnified party and any other party
represented by such counsel in such proceeding; and provided further that the
failure of any Indemnified Party to give notice as provided herein shall not
relieve the Indemnifying Party of its obligations under this Section 4, except
to the extent that such failure to give notice shall materially adversely affect
the Indemnifying Party in the defense of any such claim or any such litigation.
No


                                       10
<PAGE>

Indemnifying Party, in the defense of any such claim or litigation, shall,
except with the consent of each Indemnified Party, consent to entry of any
judgment or enter into any settlement that does not include as an unconditional
term thereof the giving by the claimant or plaintiff therein, to such
Indemnified Party, of a release from all liability in respect to such claim or
litigation.

          4.6  INFORMATION BY HOLDER.  The Holder or Holders of Registrable
Securities included in any registration shall furnish to the Company such
information regarding such Holder or Holders and the distribution proposed by
such Holder or Holders as the Company may reasonably request in writing and as
shall be required in connection with any registration, qualification, or
compliance referred to in this Section 4.

          4.7  TRANSFER OF REGISTRATION RIGHTS.  The rights contained in this
Section 4 to cause the Company to register the Registrable Securities, may be
assigned or otherwise conveyed to any affiliate (as such term is defined in Rule
405 under the 1933 Act) of Purchaser who is a transferee or assignee of Shares
or Registrable Securities, or to a third party acquiring all of the Registrable
Securities, each of whom shall be considered a "Holder" for purposes of this
Section 4, provided that the Company is given written notice by Purchaser, at
the time of or within a reasonable time after said transfer, stating the name
and address of said transferee or assignee and identifying the securities with
respect to which such registration rights are being assigned, and provided that
any such transferee agrees in writing to be bound by the provisions of this
Section 4.

          4.8  DELAY OF REGISTRATION.  No Holder shall have any right to obtain
or seek an injunction restraining or otherwise delaying any such registration as
the result of any controversy that might arise with respect to the
interpretation or implementation of this Section 4.

          4.9  RULE 144 REPORTING.  With a view to making available to the
Holders the benefits of certain rules and regulations of the SEC which may
permit the sale of the Registrable Securities to the public without
registration, the Company agrees to use its best efforts to:

               (a)  Make and keep public information available, in accordance
with subsection (c) of Rule 144 under the 1933 Act ("Rule 144") or any similar
or analogous rule promulgated under the 1933 Act, as long as Registrable
Securities are outstanding;

               (b)  File with the SEC, in a timely manner, all reports and other
documents required of the Company under the 1933 Act and 1934 Act;

               (c)  So long as a Holder owns any Registrable Securities, furnish
to such Holder forthwith upon request: a written statement by the Company as to
its compliance with the reporting requirements of Rule 144 and of the 1934 Act;
a copy of the most recent annual or quarterly report of the Company; and such
other reports and documents as a Holder may reasonably request in availing
itself of any rule or regulation of the SEC allowing it to sell any such
securities without registration.


                                       11
<PAGE>

               (d)  Take all such action (including without limitation the
furnishing of the information described in Rule 144(d)(4)) as may be necessary
or helpful to facilitate a sale of Registrable Securities by a Holder to a
"qualified institutional buyer," as such term is defined in Rule 144A of the
1933 Act.

          4.10 "MARKET STAND-OFF" AGREEMENT.  Purchaser hereby agrees that,
except for Registrable Securities being sold pursuant to this Section 4, during
the ninety (90)-day period following the effective date of a registration
statement of the Company filed under the 1933 Act, it shall not, to the extent
requested by the Company or any underwriter, sell or otherwise transfer or
dispose of any Common Stock of the Company held by it at any time during such
period; PROVIDED, HOWEVER, that:

               (a)  Such agreement shall be applicable only to registration
statements (other than on Form S-8) of the Company which cover Common Stock (or
other securities) to be sold on its behalf to the public;

               (b)  Such Agreement shall be applicable only if Purchaser holds
at least one percent (1%) of the Common Stock of the Company then outstanding;
and

               (c)  All executive officers and directors of the Company, and all
other corporate partners of the Company similarly situated, enter into similar
agreements.

     The Company may impose stop-transfer instructions with respect to
securities subject to the foregoing restriction until the end of such period.
The agreement of the Purchaser set forth in this Section 4.10 shall lapse five
(5) years after the Closing Date provided that Purchaser is not at such time an
affiliate of the Company (as defined in Rule 405 under the 1933 Act), in which
case such restrictions shall lapse at such time as Purchaser ceases to be an
affiliate.

     5.   CONDITIONS TO CLOSING.

          5.1  CONDITIONS TO OBLIGATIONS OF PURCHASER.  Purchaser's obligation
to purchase the Shares at the Closing is subject to the fulfillment, at or prior
to the Closing, of all of the following conditions, any of which may be waived
by Purchaser:

               (a)  REPRESENTATIONS AND WARRANTIES TRUE; PERFORMANCE OF
OBLIGATIONS.  The representations and warranties made by the Company in
Section 2 hereof, to the extent qualified as to materiality, shall be true and
correct in all respects and, to the extent not so qualified, shall be true and
correct in all material respects on the date of the Closing with the same force
and effect as if they had been made on and as of said date; the business and
assets of the Company shall not have been adversely affected in any material way
prior to the Closing; and the Company shall have performed and complied with all
obligations, agreements and conditions herein required to be performed by it on
or prior to the Closing.


                                       12
<PAGE>

               (b)  OPINION OF THE COMPANY'S COUNSEL.  Purchaser shall have
received from Cooley Godward LLP, counsel to the Company, an opinion letter
substantially in the form attached hereto as Exhibit B, addressed to it, dated
the date of the Closing.

               (c)  PROCEEDINGS AND DOCUMENTS.  All corporate and other
proceedings in connection with the transactions contemplated at the Closing
hereby and all documents and instruments incident to such transactions shall be
reasonably satisfactory in substance and form to Purchaser, and Purchaser shall
have received all such counterpart originals or certified or other copies of
such documents as they may reasonably request.

               (d)  QUALIFICATIONS, LEGAL INVESTMENT.  All authorizations,
approvals, or permits, if any, of any governmental authority or regulatory body
of the United States or of any state that are required in connection with the
lawful sale and issuance of the Shares pursuant to this Agreement shall have
been duly obtained and shall be effective on and as of the Closing.  No stop
order or other order enjoining the sale of the Shares shall have been issued and
no proceedings for such purpose shall be pending or, to the knowledge of the
Company, threatened by the SEC or any commissioner of corporations or similar
officer of any other state having jurisdiction over this transaction.  At the
time of the Closing, the sale and issuance of the Shares shall be legally
permitted by all laws and regulations to which Purchaser and the Company are
subject.

               (e)  COMPLIANCE CERTIFICATE.  The Company shall have delivered to
Purchaser a Certificate, executed by the President of the Company, dated the
Closing Date, certifying to (i) the fulfillment of the conditions specified in
subparagraphs (a) and (d) of this Subsection 5.1. and (ii) the incumbency of the
officers of the Company executing this Agreement and the other instruments
delivered by the Purchaser upon the Closing.

          5.2  CONDITIONS TO OBLIGATIONS OF THE COMPANY.  The Company's
obligation to issue and sell the Shares at the Closing is subject to the
fulfillment, on or prior to the Closing, of the following conditions, any of
which may be waived by the Company:

               (a)  REPRESENTATIONS AND WARRANTIES TRUE.  The representations
and warranties made by Purchaser in Section 3 hereof shall be true and correct
at the date of the Closing, with the same force and effect as if they had been
made on and as of said date.

               (b)  PERFORMANCE OF OBLIGATIONS.  Purchaser shall have performed
and complied with all obligations, agreements and conditions herein required to
be performed or complied with by it on or before the Closing.

               (c)  QUALIFICATIONS, LEGAL INVESTMENT.  All authorizations,
approvals, or permits, if any, of any governmental authority or regulatory body
of the United States or of any state that are required in connection with the
lawful sale and issuance of the Shares pursuant to this Agreement shall have
been duly obtained and shall be effective on and as of the Closing.  No stop
order or other order enjoining the sale of the Shares shall have been issued and
no proceedings for such purpose shall be pending or, to the knowledge of the
Company, threatened


                                       13
<PAGE>

by the SEC or any commissioner of corporations or similar officer of any state
having jurisdiction over this transaction.  At the time of the Closing, the sale
and issuance of the Shares shall be legally permitted by all laws and
regulations to which Purchaser and the Company are subject.

     6.   MISCELLANEOUS.

          6.1  GOVERNING LAW.  This Agreement shall be governed by and construed
under the laws of the State of Delaware, without regard to the principles
regarding conflicts of laws of such State.

          6.2  SURVIVAL.  The representations, warranties, covenants, and
agreements made herein shall survive any investigation made by Purchaser and the
closing of the transactions contemplated hereby.  All statements as to factual
matters contained in any certificate or other instrument delivered by or on
behalf of the Company pursuant hereto or in connection with the transactions
contemplated hereby shall be deemed to be representations and warranties by the
Company hereunder as of the date of such certificate or instrument.

          6.3  SUCCESSORS AND ASSIGNS.  Except as otherwise expressly provided
herein, the provisions hereof shall inure to the benefit of, and be binding
upon, the successors, assigns, heirs, executors, and administrators of the
parties hereto (including any permitted transferee of the Shares).

          6.4  ENTIRE AGREEMENT.  This Agreement, the Exhibits hereto, and the
other documents delivered pursuant hereto constitute the full and entire
understanding and agreement among the parties with regard to the subjects hereof
and no party shall be liable or bound to any other party in any manner by any
representations, warranties, covenants, or agreements except as specifically set
forth herein or therein.  Nothing in this Agreement, express or implied, is
intended to confer upon any party, other than the parties hereto and their
respective successors and assigns, any rights, remedies, obligations, or
liabilities under or by reason of this Agreement, except as expressly provided
herein.

          6.5  SEPARABILITY.  In case any provision of this Agreement shall be
invalid, illegal, or unenforceable, it shall to the extent practicable, be
modified so as to make it valid, legal and enforceable and to retain as nearly
as practicable the intent of the parties, and the validity, legality, and
enforceability of the remaining provisions shall not in any way be affected or
impaired thereby.

          6.6  AMENDMENT AND WAIVER.  Any term of this Agreement may be amended
and the observance of any term of this Agreement may be waived (either generally
or in a particular instance, either retroactively or prospectively, and either
for a specified period of time or indefinitely), with the written consent of the
Company and the holders of not less than a majority-in-interest of the aggregate
of outstanding Shares.  Any amendment or waiver effected in accordance with this
paragraph shall be binding upon Purchaser, each future holder of the Shares, and
the Company.  Upon the effectuation of each such amendment or waiver, the


                                       14
<PAGE>

Company shall promptly give written notice thereof to the record holders of the
Shares who have not previously consented thereto in writing, if any.

          6.7  DELAYS OR OMISSIONS.  No delay or omission to exercise any right,
power, or remedy accruing to Purchaser or any subsequent holder of any Shares
upon any breach, default or noncompliance of the Company under this Agreement,
shall impair any such right, power, or remedy, nor shall it be construed to be a
waiver of any such breach, default or noncompliance, or any acquiescence
therein, or of any similar breach, default or noncompliance thereafter
occurring.  It is further agreed that any waiver, permit, consent, or approval
of any kind or character on Purchaser's part of any breach, default or
noncompliance under this Agreement or any waiver on Purchaser's part of any
provisions or conditions of this Agreement must be in writing and shall be
effective only to the extent specifically set forth in such writing, and that
all remedies, either under this Agreement, by law, or otherwise afforded to
Purchaser, shall be cumulative and not alternative.

          6.8  NOTICES, ETC.  All notices and other communications required or
permitted hereunder shall be in writing and shall be deemed effectively given
(a) upon personal delivery, (b) on the first business day after receipted
delivery to a courier service which guarantees next business-day delivery, under
circumstances in which such guaranty is applicable, or (c) on the earlier of
delivery or five (5) business days after mailing by United States certified by
mail, postage and fees prepaid, to the appropriate party at the address set
forth below or to such other address as the part so notifies the other in
writing:

               (a)  if to the Company, to:

                    COCENSYS, INC.
                    213 Technology Drive
                    Irvine, CA 92718
                    Attention:  President and Chief Executive Officer
                    Fax:(714) 753-6141

                    with a copy to:

                    COOLEY GODWARD LLP
                    5 Palo Alto Square
                    3000 El Camino Real
                    Palo Alto, CA 94306-2155
                    Attention:  Alan C. Mendelson, Esq.
                    Fax:(415) 857-0663


                                       15
<PAGE>

                    if to Purchaser, to:

                    AMERICAN HOME PRODUCTS CORPORATION
                    5 Giralda Farms
                    Madison, NJ  07940
                    Attention: Senior Vice President and General Counsel

          Notwithstanding the foregoing, all notices and other communications to
an address outside of the United States shall be sent by telecopy and confirmed
in writing to be sent by first class mail.

          6.9  FINDER'S FEES.

               (a)  The Company  represents and warrants that it has retained no
finder or broker in connection with the transactions contemplated by this
Agreement and  hereby agrees to indemnify and to hold harmless of and from any
liability for any commission or compensation in the nature of a finder's fee to
any broker or other person or firm (and the costs and expenses of defending
against such liability or asserted liability) for which the Company or any of
its employees or representatives is responsible.

               (b)  Purchaser  represents and warrants that it has retained no
finder or broker in connection with the transactions contemplated by this
Agreement, and  hereby agrees to indemnify and to hold the Company harmless of
and from any liability for any commission or compensation in the nature of a
finder's fee to any broker or other person or firm (and the costs and expenses
of defending against such liability or asserted liability) for which Purchaser
or any of its employees or representatives are responsible.

          6.10 FEES AND EXPENSES.  Each party agrees to pay all its own
fees, costs and expenses, including legal and accounting fees, relating to the
negotiation, execution, delivery and performance this Agreement and the
transactions contemplated hereby.  If legal action is brought by, or on behalf
of, Purchaser to enforce or interpret this Agreement, the prevailing party shall
be entitled to recover its attorneys' fees and legal costs in connection
therewith.

               6.11 INFORMATION CONFIDENTIAL.  Purchaser acknowledges that
certain information received by it pursuant hereto is confidential and for
Purchaser's use only, and it will refrain from using such information or
reproducing, disclosing, or disseminating such information to any other person
(other than its employees, affiliates, agents, or partners having a need to know
the contents of such information and its attorneys, in each case who agree to be
bound by this Section 6.11), except in connection with the exercise of rights
under this Agreement, unless such information (i) is or becomes, through no
fault of Purchaser, available to the public generally; (ii) was already known by
Purchaser, as demonstrated by competent evidence, at the time of its receipt
from the Company; (iii) is obtained by Purchaser from a third party legally free
to disclose such information; or (iv) Purchaser is required by a governmental
body or court of competent jurisdiction to disclose such information.


                                       16
<PAGE>

          6.12  TITLES AND SUBTITLES.  The titles of the sections and
subsections of this Agreement are for convenience of reference only and are not
to be considered in construing this Agreement.

          6.13  COUNTERPARTS.  This Agreement may be executed in any number of
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one instrument.


                                       17
<PAGE>

      The foregoing Agreement is hereby executed as of the date first above
written.


COCENSYS, INC.                         AMERICAN HOME PRODUCTS CORPORATION
213 Technology Drive                   5 Giralda Farms
Irvine, CA 92718                       Madison, NJ  07940



By:                                    By:
   ---------------------------            ----------------------------
F. Richard Nichol                      Name:
President and Chief Executive Officer  Title:


                                          18
<PAGE>

                                       EXHIBITS


Exhibit A  -  CERTIFICATE OF DESIGNATION
Exhibit B  -  OPINION OF COMPANY'S COUNSEL


<PAGE>

                                      EXHIBIT A


                              CERTIFICATE OF DESIGNATION

<PAGE>

                   CERTIFICATE OF POWERS, DESIGNATION, PREFERENCES,
                                RIGHTS AND LIMITATIONS

                                          OF

                         SERIES C CONVERTIBLE PREFERRED STOCK

                                          OF

                                    COCENSYS, INC.

                           (Pursuant to Section 151 of the
                          Delaware General Corporation Law)

        COCENSYS, INC., a corporation organized and existing under the General
Corporation Law of the State of Delaware (hereinafter called the "Corporation"),
hereby certifies that the following resolution was adopted by the Board of
Directors of the Corporation as required by Section 151 of the General
Corporation Law at a meeting duly called and held on April 29, 1997:

         RESOLVED, that pursuant to the authority granted to and vested in the
    Board of Directors of the Corporation in accordance with the provisions of
    its Amended and Restated Certificate of Incorporation, the Board of
    Directors hereby creates a series of Preferred Stock, par value $.001 per
    share, of the Corporation and hereby states the designation and number of
    shares, and fixes the relative rights, preferences and limitations thereof
    (in addition to the provisions set forth in the Restated Certificate of
    Incorporation of the Corporation, which are applicable to the Preferred
    Stock of all classes and series), as follows:

    Series C Convertible Preferred Stock:

         SECTION 1. DESIGNATION AND AMOUNT. One Hundred Thousand (100,000)
    shares of Preferred Stock, $.001 par value, are designated "Series C
    Convertible Preferred Stock" with the rights, preferences, privileges and
    restrictions specified herein (the


                                          1.
<PAGE>

    "Series C Preferred Stock"). Such number of shares may be not increased or
    decreased without the consent of the holder.

         SECTION 2. DIVIDENDS AND DISTRIBUTIONS. The holders of the Series C
    Preferred Stock shall be entitled to receive, when, as and if declared by
    the Board of Directors, out of funds legally available therefor, dividends
    at the rate per share equal to any dividend declared or paid per share to
    the Common Stock of the Corporation ("Common Stock"). The right to such
    dividends on the Series C Preferred Stock shall be non-cumulative.

         SECTION 3. VOTING RIGHTS. Except as set forth herein. or as otherwise
    provided by law, holders of Series C Preferred Stock shall have no special
    voting rights and their consent shall not be required (except to the extent
    they are entitled to vote with holders of Common Stock as set forth herein)
    for taking any corporate action.

         SECTION 4. LIQUIDATION PREFERENCE. In the event of any liquidation,
    dissolution or winding up of the Corporation, either voluntary or
    involuntary (a "Liquidation Event"), the holders of the Series C Preferred
    Stock shall be entitled to receive, prior and in preference to any
    distribution of any of the assets or surplus funds of the Corporation to
    the holders of the Common Stock or Junior Preferred Stock of the
    Corporation, an amount per share (as adjusted for any combinations,
    consolidations, stock distributions or stock dividends with respect to such
    shares) equal to the quotient of (a) $5,000,000 divided by (b) the number
    of shares of Series C Preferred Stock issued and outstanding as of the date
    of such Liquidation Event. If upon the occurrence of such Liquidation
    Event, the assets and funds thus distributed among the holders of the
    Series C Preferred Stock shall be insufficient to permit the payment to
    such holders of the full aforesaid preferential amount, then the entire 
    assets and funds of the Corporation legally available for distribution shall
    be distributed among the holders of the Series C Preferred Stock in
    proportion to the shares of Series C Preferred Stock then held by them.

         SECTION 5. CONVERSION. Subject to the limitations set forth in
    Subsection (B) below, the Series C Preferred Stock shall convert only as
    follows:

         (A) CONVERSION AT HOLDER'S OPTION. At any time after May _, 1999, the
    Series C Preferred Stock shall be convertible, in whole or in part, on a
    maximum of three occasions, at the


                                          2.
<PAGE>

    option of the holder, into such number of fully paid and nonassessable
    shares of Common Stock equal to the quotient of (a) the product of $50 and
    the number of shares of Series C Preferred Stock being converted, divided
    by (b) the Conversion Price.

    The "Conversion Prices" shall be equal to the greater of:

         (i)  $[5.43] or

         (ii) the lesser of:

              (x)  the Future Market Price x 0.80 or

              (y)  $[7.76];

    PROVIDED, HOWEVER, that if the Future Market Price is less than $[3.88],
    the Conversion Price shall be $[4.37].

    The "Future Market Price" set forth above shall be the average closing
    price of the Common Stock for the period commencing on the 23rd trading day
    prior to the date upon which the holder delivers notice to the Corporation
    of such conversion (each, a "Conversion Date") and ending on the third
    trading day prior to the Conversion Date, as reported in the WALL STREET
    JOURNAL, WESTERN EDITION.

         (B) ADJUSTMENTS FOR COMBINATIONS OR SUBDIVISIONS OF COMMON STOCK. In
    the event the Corporation at any time or from time to time shall declare or
    pay any dividend on the Common Stock payable in Common Stock or in any
    right to acquire Common Stock, or shall effect a subdivision of the
    outstanding shares of Common Stock into a greater number of shares of
    Common Stock (by stock split, reclassification or otherwise), or in the
    event the outstanding shares of Common Stock shall be combined or
    consolidated, by reclassification or otherwise, into a lesser number of
    shares of Common Stock, then the maximum and minimum number of shares of
    Common Stock into which the Series C Preferred Stock may be converted,
    shall be proportionately decreased or increased, as appropriate.

         (C) MECHANICS OF CONVERSION. Before any holder of Series C Preferred
    Stock shall be entitled to receive shares of Common Stock, he shall
    surrender the certificate or certificates thereof, duly endorsed, at the
    office of the Corporation or of any transfer agent for such stock, and
    shall state therein the name or


                                          3.
<PAGE>

    names in which he wishes the certificate or certificates shares of Common
    Stock to be issued. The Corporation shall, as soon as practicable
    thereafter, issue and deliver at such office to such holder of Series C
    Preferred Stock, a certificate or certificates for the number of shares of
    Common Stock to which he shall be entitled as aforesaid. Such conversion
    shall be deemed to have been made on the Conversion Date, and the person or
    persons entitled to receive the shares of Common Stock issuable upon such
    conversion shall be treated for all purposes as the record holder or
    holders of such shares of Common Stock on such date.

         (D) RESERVATION OF STOCK ISSUABLE UPON CONVERSION. The Corporation
    shall at all times reserve and keep available out of its authorized but
    unissued shares of Common Stock, solely for the purpose of effecting the
    conversion of the shares of the Series C Preferred Stock, such number of
    its shares of Common Stock as shall from time to time be sufficient to
    effect the conversion of all outstanding shares of the Series C Preferred
    Stock.

         (E) FRACTIONAL SHARES. No fractional share shall be issued upon the
    conversion of any share or shares of Series C Preferred Stock. All shares
    of Common Stock (including fractions thereof) issuable upon conversion of
    Series C Preferred Stock shall be aggregated for purposes of determining
    whether the conversion would result in the issuance of any fractional
    share. If, after the aforementioned aggregation, the conversion would
    result in the issuance of a fraction of a share of Common Stock, the
    Corporation shall, in lieu of issuing any fractional share, pay the holder
    otherwise entitled to such fraction a sum in cash equal to the closing
    price of the Common Stock on the date of conversion, multiplied by such
    fraction.

         (F) REORGANIZATION, RECLASSIFICATION, CONSOLIDATION, MERGER OR SALE.
    If any (i) reorganization of the capital stock of the Corporation, (ii)
    consolidation or merger of the Corporation in which the Corporation is not
    the surviving corporation, or (iii) sale of all or substantially all of the
    Corporation's assets to another corporation (each, an "Event") shall be
    effected in such a way that holders of Common Stock shall be entitled to
    receive securities, cash or other assets or property, the first Conversion
    Date shall be accelerated to the date immediately preceding such Event, or
    such other date necessary to assure that any holder of Series C Preferred
    Stock receives such shares of stock, securities or other assets or property
    as may be issued or payable with respect to or in exchange for shares of
    Common Stock.


                                          4.
<PAGE>

         SECTION 6. NO REDEMPTION. The shares of Series C Preferred Stock shall
    not be redeemable.

         SECTION 7. AMENDMENT. The Restated Certificate of Incorporation of the
    Corporation shall not be amended in any manner which would materially alter
    or change the powers, preferences or special rights of the Series C
    Preferred Stock so as to affect them adversely without the affirmative vote
    of the holders of at least two-thirds of the outstanding shares of Series C
    Preferred Stock, voting together as a single class.


                                          5.
<PAGE>

    IN WITNESS WHEREOF the undersigned have executed this certificate as of
May ___, 1997.

                                       ---------------------------------------
                                       F. Richard Nichol, Ph.D.
                                       President and Chief Executive Officer

                                       ---------------------------------------
                                       Alan C. Mendelson
                                       Secretary


                                          6.
<PAGE>

                                      EXHIBIT B


                             OPINION OF COMPANY'S COUNSEL


<PAGE>

                                      EXHIBIT B
                                   FORM OF OPINION


May ___, 1997


American Home Products Corporation
5 Giralda Farms
Madison, NJ  07940

RE:  SALE AND PURCHASE OF COCENSYS, INC. SERIES C PREFERRED STOCK

Gentlemen:

We have acted as counsel for CoCensys, Inc., a Delaware corporation (the
"Company"), in connection with the issuance and sale of 100,000 shares of the
Company's Series C Preferred Stock to American Home Products Corporation, a
Delaware corporation ("Purchaser"), pursuant to the terms of that certain Stock
Purchase Agreement, dated May ___, 1997, by and between the Company and
Purchaser (the "Agreement").  The shares of Company Series C Preferred Stock
issued to Purchaser at the closing (the "Closing") are referred to herein as the
"Shares".  We are rendering this opinion pursuant to Section 5.1(b) of the
Agreement.  Except as otherwise defined herein, capitalized terms used but not
defined herein have the respective meanings given to them in the Agreement.

In connection with this opinion, we have examined and relied upon the
representations and warranties as to factual matters contained in and made
pursuant to the Agreement by the parties thereto and originals or copies
certified to our satisfaction, of such records, documents, certificates,
opinions, memoranda and other instruments as in our judgment are necessary or
appropriate to enable us to render the opinion expressed below.  Where we render
an opinion "to the best of our knowledge" or concerning an item "known to us" or
our opinion otherwise refers to our knowledge, it is based solely upon (i) an
inquiry of attorneys within this firm who perform legal services for the
Company, (ii) receipt of a certificate executed by an officer of the Company
covering such matters, and (iii) such other investigation, if any, that we
specifically set forth herein.

In rendering this opinion, we have assumed:  the genuineness and authenticity of
all signatures on original documents; the authenticity of all documents
submitted to us as originals; the conformity to originals of all documents
submitted to us as copies; the accuracy, completeness and authenticity of
certificates of public officials; and the due authorization, execution and
delivery of all documents where authorization, execution and delivery are
prerequisites to the effectiveness of such documents (except the due
authorization, execution and delivery of the Agreement by the Company).  We have
also assumed: that all individuals executing and delivering documents had the
legal capacity to so execute and deliver; that you have received all documents
you were to receive under the Agreement; that the Agreement is an obligation
binding upon you; if you are a corporation or other entity, that you have filed
any required California franchise or income tax returns and have paid any
required


<PAGE>

American Home Products Corporation
May ___, 1997
Page 2


California franchise or income taxes; and that there are no extrinsic agreements
or understandings among the parties to the Agreement that would modify or
interpret the terms of the Agreement or the respective rights or obligations of
the parties thereunder.

Our opinion is expressed only with respect to the federal laws of the United
States of America and the laws of the State of California and the General
Corporation Law of the State of Delaware.  We express no opinion as to whether
the laws of any particular jurisdiction apply, and no opinion to the extent that
the laws of any jurisdiction other than those identified above are applicable to
the subject matter hereof.  We are not rendering any opinion as to compliance
with any antifraud law, rule or regulation relating to securities, or to the
sale or issuance thereof.

Our opinions in paragraphs 5, 6 and 7 below as they relate to the Conversion
Shares are based upon the hypotheses that the Shares are convertible, and are
being converted, on the date hereof.

On the basis of the foregoing, in reliance thereon and with the foregoing
qualifications, we are of the opinion that:

    1.   The Company has been duly incorporated and is a validly existing
corporation in good standing under the laws of the State of Delaware.

    2.   The Company has the requisite corporate power to own or lease its
property and assets and to conduct its business as it is currently being
conducted and, to the best of our knowledge, is qualified as a foreign
corporation to do business in each jurisdiction in the United States in which
the ownership of its property or the conduct of its business requires such
qualification and where any statutory fines or penalties or any corporate
disability imposed for the failure to qualify would materially or adversely
affect the Company, its assets, financial condition or operations.

    3.   The Agreement has been duly and validly authorized, executed and
delivered by the Company and constitutes a valid and binding agreement of the
Company in accordance with its terms, except as rights to indemnity thereunder
may be limited by applicable laws and except as enforcement may be limited by
applicable bankruptcy, insolvency, reorganization, arrangement, moratorium or
other similar laws affecting creditors' rights, and subject to general equity
principles and to limitations on availability of equitable relief, including
specific performance.

    4.   The Shares have been duly authorized and, upon issuance and delivery
in accordance with the terms of the Agreement, will be validly issued, fully
paid and nonassessable.  The


<PAGE>

American Home Products Corporation
May ___, 1997
Page 3

Conversion Shares have been duly authorized and, upon issuance and delivery in
accordance with the Certificate of Designation, will be validly issued, fully
paid and nonassessable.

    5.   The issuance and sale of the Shares and the issuance of the Conversion
Shares, in each case as contemplated by the Agreement, do not violate any
provision of the Company's Amended and Restated Certificate of Incorporation or
Bylaws and do not violate or contravene (a) any governmental statute, rule or
regulation applicable to the Company or (b) any order, writ, judgment,
injunction, decree, determination or award which has been entered against the
Company and of which we are aware, the violation or contravention of which would
materially and adversely affect the Company, its assets, financial condition or
operations.

    6.   All consents, approvals, authorizations, or orders of, and filings,
registrations, and qualifications with any regulatory authority or governmental
body in the United States required for the issuance and sale of the Shares and
the issuance of the Conversion Shares, in each case as contemplated by the
Agreement, have been made or obtained.

    7.   The issuance and sale of the Shares and the issuance of the Conversion
Shares, in each case as contemplated by the Agreement is exempt from the
registration requirements of the Securities Act of 1933, as amended.

This opinion is intended solely for your benefit and is not to be made available
to or be relied upon by any other person, firm, or entity without our prior
written consent.

Very truly yours,

COOLEY GODWARD LLP



By ________________________