<PAGE>

Portions of this Exhibit have been omitted pursuant to a request for
confidential treatment.  The omitted portions, marked by a * and [   ], have
been separately filed with the Commission.

                              COPROMOTION AGREEMENT


     This Copromotion Agreement effective June 1, 1996, between WYETH-AYERST
LABORATORIES, 555 E. Lancaster Avenue, St.  Davids, PA 19087 ("Wyeth-Ayerst")
and INTERNEURON PHARMACEUTICALS, INC., One Ledgemont Center, 99 Hayden Avenue,
Suite 340, Lexington, MA 02173(" Interneuron ").

     WHEREAS, Interneuron has granted Wyeth-Ayerst exclusive patent and know-how
licenses to make, have made, use and sell finished pharmaceuticals containing
dexfenfluramine hydrochloride ("REDUX") for the treatment of obesity in the
United States.  its territories and possessions and the Commonwealth of Puerto
Rico (the "Territory") pursuant to Patent and Know-How Sublicense and Supply
Agreement effective as of November 19, 1992, (the "Sublicense Agreement") as
amended pursuant to Consent and Amendment Agreement effective as of November 21,
1994 (the "Amendment Agreement"); and

     WHEREAS, pursuant to Paragraph 6.4.2 of the Sublicense Agreement,
Interneuron retained copromotion rights to REDUX in the Territory, the details
of such copromotion, including the practitioner categories to whom Interneuron
would promote REDUX to be agreed to by Wyeth-Ayerst and Interneuron;

     NOW, THEREFORE, in consideration of the foregoing and the mutual promises
and covenants herein contained, it is hereby agreed as follows:

1.  Definitions

     When used in this Agreement, the following terms shall have the following
respective meanings:
<PAGE>

The information below marked by * and [ ] has been omitted pursuant to a request
for confidential treatment.  The omitted portion has been separately filed with
the Commission.

     (a) "Contract Month" shall mean each one month period during the term of
this Agreement beginning June 1, 1996.

     (b) "Contract Quarter" shall mean each three month period during the term
of this Agreement beginning June 1, 1996, provided, however the First Contract
Quarter shall end June 30, 1996.

     (c) "Contract Year" shall mean each one year period during the term of this
Agreement beginning June 1, 1996.

     (d) "Copromotion" shall mean those activities normally undertaken by a 
pharmaceutical company's sales force to implement marketing plans and strategies
aimed at encouraging the appropriate use of a particular prescription
pharmaceutical product.  

     (e) "Detail" shall mean a one-on-one, face-to-face meeting, in an
individual or group practice setting, between one or more physician prescribers
and one Interneuron professional representative during which product information
is communicated that is in addition to any discussion regarding the physician's
sample needs.  When used as a verb, "Detail" shall mean to engage in a Detail.

     (f) "First Position Detail" shall mean [__________________________________]

     (9) "REDUX Sales Force" shall mean up to thirty-three (33) regular, full-
time employees of Interneuron, thirty (30) of whom shall be employed and trained
as professional representatives to Detail REDUX to the Interneuron Categories
and three (3) of whom shall provide field sales management for the REDUX Sales
Force.

     (h) "Sample" shall mean a unit of REDUX that is not intended to be sold and
is intended to promote the sale of REDUX.


                                       -2-
<PAGE>

The information below marked by * and [ ] has been omitted pursuant to a request
for confidential treatment.  The omitted portion has been separately filed with
the Commission.

2.   COPROMOTION BY INTERNEURON

     Interneuron shall copromote REDUX in the Territory by causing its REDUX
Sales Force to Detail to individual physicians, and small group practices of
physicians, who are bariatricians, nutritionists, full time-weight management
specialists, endocrinologists and diabetologists (the "Interneuron Categories").
Attached hereto as Exhibit A is an electronic list of physicians and small group
practices which shall constitute the candidates in the Interneuron Categories. 
Exhibit A shall be periodically updated by mutual agreement of the parties.

     The Interneuron Categories shall not include national and regional weight
loss organizations, e.g.  NutriSystems, Weight Watchers, etc., which offer
multiple weight loss approaches and/or where physician supervised weight loss is
not the primary intervention.  Physicians with multi-location offices or clinics
shall not PER SE be considered to be excluded as national or regional weight
loss organizations.

     Physicians who Wyeth-Ayerst actively Details (defined as four (4) calls in
the previous twelve (12) months) on the effective date hereof may be excluded
from the Interneuron Categories.
     
[_______________________________________________________________________________
_____________]

3.   DETAILING ADDITIONAL WYETH-AYERST PRODUCTS

     During the first two (2) Contract Years, the REDUX Sales Force shall, if
Wyeth-Ayerst in its sole discretion so requests, Detail to the Interneuron
Categories additional


                                       -3-
<PAGE>

The information below marked by * and [ ] has been omitted pursuant to a request
for confidential treatment.  The omitted portion has been separately filed with
the Commission.

Wyeth-Ayerst products at the time of Details for REDUX.  Wyeth-Ayerst shall
provide promotional materials and samples to Interneuron at no cost for such
detailing.  Upon mutual agreement, physicians other than those in the
Interneuron Categories may be detailed by the REDUX Sales Force hereunder.  For
detailing additional Wyeth-Ayerst products in the first Contract Year,
Interneuron shall receive no compensation other than the Interneuron sales force
subsidy provided in Section 4(a).  For such additional detailing in the second
Contract Year, Wyeth-Ayerst shall pay Interneuron [$___] per full product
Detail.

4.   INTERNEURON SALES FORCE SUBSIDY AND PROFIT SHARING

          (a) SALES FORCE SUBSIDY.  For the first two (2) Contract Years, Wyeth
Ayerst shall pay the following percentages of Interneuron's actual REDUX Sales
Force costs:  

          Year 1 - 100% 
          Year 2 - 50%

Provided, however, that the REDUX Sales Force shall not exceed thirty-three (33)
regular, full-time employees and Wyeth-Ayerst's payment obligation shall be
based on the above percentages of Interneuron actual REDUX Sales Force costs,
including, but not limited to, payroll, fringe benefits, travel expenses, sales
training costs, commissions and bonuses documented by Interneuron to Wyeth-
Ayerst's reasonable satisfaction, not in excess of [$_____________] per
representative per Contract Year.  Interneuron shall provide Wyeth-Ayerst within
ten (10) days of the end of each Contract Month a monthly schedule of REDUX
Sales Force expenses along with a roster of sales people setting


                                       -4-
<PAGE>

The information below marked by * and [ ] has been omitted pursuant to a request
for confidential treatment.  The omitted portion has been separately filed with
the Commission.

forth their dates of hire.  Within thirty (30) days of receipt of such schedule,
Wyeth-Ayerst will pay the amount provided for herein.

     (b)  PROFIT SHARING.  For the first three (3) Contract Years, Wyeth-Ayerst
shall pay Interneuron the following percentages of REDUX Brand Profit, as
defined below, by physician category, for physicians or small groups in the
Interneuron Categories detailed by the REDUX Sales Force at least [_________]
times each Contract Year:

               Bariatricians
               Nutritionists                 Endocrinologists
               Weight Mgt. Specialists       Diabetologists
               -----------------------       --------------

          Year 1    [______________________________]
          Year 2    [______________________________]
          Year 3    [______________________________]

     Profit shall mean REDUX Net Sales (including any and all sales of REDUX by
Wyeth-Ayerst made prior to June 1, 1996) in the Interneuron Categories less cost
of goods sold, distribution costs, the costs of detail aids and other
promotional materials and samples for the REDUX Sales Force and allocated costs
for direct advertising and promotion expenses.  REDUX Net Sales shall mean the
gross amount invoiced for REDUX less transportation charges or allowances, if
any, included in such price; trade, quantity or cash discounts, service
allowances and broker's or agent's commissions, if any, allowed or paid; credits
or allowances, if any, given or made on account of price adjustments, returns,
bad debts (actual write-offs only), off-invoice promotional discounts, rebates,
any and all federal, state or local government rebates whether in existence now
or enacted at any time during the Term of this Agreement (e.g. HCFA or


                                       -5-
<PAGE>

The information below marked by * and [ ] has been omitted pursuant to a request
for confidential treatment.  The omitted portion has been separately filed with
the Commission.

Medicaid rebates), rejections, recalls or REDUX destruction (voluntarily made or
requested or made by an appropriate government agency, sub-division or
department); and any tax, excise or governmental charge upon or measured by the
production, sale, transportation, delivery, or use of REDUX.  Net Sales shall be
measured for the Interneuron Categories by utilizing the Xponent Rx database
supplemented by additional data sources appropriate in Wyeth-Ayerst's reasonable
judgment to accurately measure direct and indirect sales to dispensing
physicians in the Interneuron Categories.  The above listed deductions from Net
Sales used to compute Profit hereunder shall be at cost.  No intercompany
transfer profit shall be considered in calculating Profit hereunder.  All
expenses considered under this profit sharing calculation will be applied using
Generally Accepted Accounting Principals ("GAAP").  Direct marketing expenses
shall not include internal overheads.  Cost of goods sold and distribution
expenses for Redux are currently estimated to average [____%] of Net Sales. 
Direct advertising and promotion expenses will be allocated based on Net Sales.

     (c)  MAXIMUM INTERNEURON COMBINED SALES FORCE SUBSIDY AND PROFIT SHARING.
Wyeth-Ayerst's combined sales force subsidy and profit sharing payments to
Interneuron shall not exceed $10,000,000 in any Contract Year.

     (d)  Profit sharing payments to Interneuron under this Section shall be
made on a Contract Quarter basis within [______________] days of the end of each
Contract Quarter together with supporting calculations.  If the IMS data base is
not available to be used for the profit sharing calculation, Wyeth will use
reasonable estimates based on historical sales to perform the required profit
sharing calculation.  [_______] percent of


                                       -6-
<PAGE>

each quarterly payment for the first three (3) Contract Quarters of each
Contract Year shall be retained by Wyeth-Ayerst and remitted, as adjusted, with
the payment for the Fourth Contract Quarter which shall reflect all adjustments
to the sales force subsidy and profit sharing payments to Interneuron for the
Contract Year.

5.   MARKETING, PRICING AND SALES FORCE

          (a) Wyeth-Ayerst shall have the sole authority for the pricing,
marketing strategy and tactics for REDUX.  All promotional and detailing
materials shall be supplied by Wyeth-Ayerst and not by Interneuron.  The
determination of the content, the quantity, and the method of distribution of
the promotional materials and literature related to REDUX shall be the sole
responsibility of Wyeth-Ayerst.  In connection with the Copromotion of REDUX,
Interneuron shall use only promotional materials and literature provided by
Wyeth-Ayerst, which shall be used only for the purposes of this Agreement and
shall be returned to Wyeth-Ayerst upon termination of this Agreement.  All
copyright and other intellectual property rights therein shall remain vested in
Wyeth-Ayerst.

     (b)  Interneuron's REDUX Sales Force shall remain exclusively under the
authority of the field sales management of Interneuron.  Interneuron shall have
full responsibility for the dissemination, of information regarding REDUX to its
REDUX Sales Force based on information provided by Wyeth-Ayerst.

     (c)  Wyeth-Ayerst shall have the sole right and responsibility for
establishing and modifying the terms and conditions with respect to the sale of
REDUX, including any terms and conditions relating to or affecting the price at
which REDUX will be sold,


                                       -7-
<PAGE>

any discount attributable to payments on receivables, distribution of REDUX,
credit to be granted or refused and the like.

     (d)  Wyeth-Ayerst shall inform Interneuron of list price increases or
decreases for REDUX in the Territory at the time such information is provided to
Wyeth-Ayerst's sales force.

     (e)  Wyeth-Ayerst shall be exclusively responsible for accepting and
filling purchase orders, billing, and returns with respect to REDUX.  If
Interneuron receives an order for REDUX, it shall make best efforts to, within
forty-eight (48) hours, transmit such order to Wyeth-Ayerst for acceptance or
rejection, which acceptance or rejection shall be at Wyeth-Ayerst's sole
discretion.  REDUX Net Sales in the final Contract Year of the term or any
extended term of this Agreement shall include sales made on all orders received
prior to the end of such Contract Year regardless of the date of acceptance or
fulfillment of such orders by Wyeth-Ayerst.

     (f)  The appointment of Interneuron hereunder, shall not create a joint
venture, or an employer-employee relationship or a principal-agent relationship
other than as specifically provided in this Agreement or in the Sublicense
Agreement or Amendment Agreement.  

     (g)  Nothing in this Agreement shall be deemed to authorize Interneuron to
act for, represent, or bind Wyeth-Ayerst or any of its affiliates other than as
specifically provided by this Agreement.

     (h)  Neither party shall have any responsibility for the hiring,
termination, compensation or benefits of the other party's employees.  No
employee or representative of a party shall have an authority to bind or
obligate the other party


                                       -8-
<PAGE>

The information below marked by * and [ ] has been omitted pursuant to a request
for confidential treatment.  The omitted portion has been separately filed with
the Commission.

to this Agreement for any sum or in any manner whatsoever, or to create or
impose any contractual or other liability on the other party without said
party's authorized written approval.

6.   TERM AND TERMINATION

     (a)  The initial term of this Agreement, unless sooner terminated
hereunder, shall be for three (3) years from June 1, 1996.  Wyeth-Ayerst shall
have the option of extending the Agreement for one (1) or two (2) years
following the initial term on terms and conditions to be agreed upon by the
parties, provided that Wyeth-Ayerst shall give Interneuron three (3) months
prior written notice of any such extension.  Interneuron may elect not to
copromote REDUX for an extended one (1) or two (2) year term.

     (b)  Either party may terminate this Agreement on thirty (30) days prior
written notice if the other party is in default and such default has not been
cured within thirty (30) days of such notice.  Wyeth-Ayerst may terminate this
Agreement on thirty (30) days notice if REDUX is withdrawn from the market and
on ninety (90) days notice if total REDUX sales do not exceed [____] million in
any Contract Year or if Interneuron generated sales do not exceed [____] million
in any Contract Year.  Interneuron may terminate at any time on six (6) months
notice.  Upon notice of termination by either Wyeth-Ayerst or Interneuron,
Wyeth-Ayerst may immediately begin to phase in its own detailing to the
physicians Interneuron is calling on.

7.   NONCOMPETITION

     During the first eighteen (18) months of this Agreement, the REDUX Sales
Force will not promote another company's product.  Thereafter, the REDUX Sales
Force will


                                       -9-
<PAGE>

not promote another company's product without Wyeth-Ayerst's prior consent which
shall not be unreasonably withheld.  Such consent is conditioned on the products
not being a weight loss product; not being competitive with other Wyeth-Ayerst
products, and that REDUX remain in First Position Detail with full product
presentation for the entire term of this Agreement.

     Interneuron shall not promote another anti-obesity drug during the term of
this Agreement.  This provision is not intended to preclude Interneuron's
copromotion of other anti-obesity drugs for Wyeth-Ayerst nor does it grant
rights to Interneuron to do so.  Interneuron shall not promote another anti-
obesity prescription drug product for one year following expiration of this
Agreement or one year following termination by Wyeth-Ayerst because of default
by Interneuron or termination by Interneuron except for default by Wyeth-Ayerst.

8.   SAMPLES 

          (a)  Samples of REDUX, to be used by Interneuron only for sampling to
Interneuron Categories, shall be sent to Interneuron at an agreed upon site and
at Wyeth-Ayerst's expense when REDUX has been descheduled as a controlled
substance.  The storage and distribution of such Samples to the REDUX Sales
Force shall be at Interneuron's expense.  Storage and distribution of such
Samples by Interneuron and sampling by the REDUX Sales Force shall be in
compliance with all federal, state and local laws and regulations and Wyeth-
Ayerst procedures concerning prescription drug samples, including but not
limited to, all laws concerning sample accountability.  Wyeth-Ayerst shall have
the right to audit Interneuron's compliance with the above-referenced laws and
regulations upon reasonable notice and at reasonable


                                      -10-
<PAGE>

times not to exceed twice a Contract Year.  For purposes of this article, such
audit may, at Wyeth-Ayerst's option, be conducted by either Wyeth-Ayerst's
employees or consultants.

     (b)  The decision to sample shall be determined by Wyeth-Ayerst, the
quantity of Samples to be provided to Interneuron and the method of
distribution, including any limitations thereon, shall be determined jointly by
the parties.

     (c)  Interneuron shall, within sixty (60) days of the end of each Contract
Quarter during the term of this Agreement, provide to Wyeth-Ayerst a summary
analysis of the number of Samples distributed to Interneuron Categories during
the Contract Quarter.

     (d)  Interneuron shall be an Authorized Distributor of Record for REDUX for
purposes of the requirements of the Prescription Drug Marketing Act of 1987 (the
"PDMA") and shall comply with the PDMA, FDA regulations and applicable state law
requirements regarding the marketing, sale and distribution of REDUX, including
but not limited to applicable wholesale drug distribution licensing guidelines
and requirements.

     (e)  Interneuron shall establish and adhere to written procedures to assure
that Interneuron and the REDUX Sales Force comply with all requirements of the
PDMA.  For this purpose, Wyeth-Ayerst shall provide for each REDUX Sales Force
employee a copy of the Wyeth-Ayerst Sample Accountability Manual, the receipt of
which shall be acknowledged and compliance agreed to by each such REDUX Sales
Force employee.  Interneuron shall notify Wyeth-Ayerst immediately upon learning
that any REDUX Samples shipped by Wyeth-Ayerst to Interneuron have been lost or
have not been received as scheduled.  In the event that Interneuron or any of
the REDUX Sales Force


                                      -11-
<PAGE>


fails to comply or causes Wyeth-Ayerst to fail to comply with applicable legal
requirements and as a direct result a civil penalty is assessed against Wyeth-
Ayerst or any of its affiliates or employees, then Interneuron shall hold
harmless and indemnify Wyeth-Ayerst, its affiliates or its employees from any
such civil penalty or other damages or losses related thereto, including
reasonable attorneys' fees unless such failure was caused by Wyeth-Ayerst.

9.   ADVERSE REACTION REPORTING AND REGULATORY MATTERS

          (a)  Interneuron shall notify Wyeth-Ayerst within twenty-four (24)
hours if Interneuron receives any notice of a serious or unexpected adverse drug
experience associated with the use of REDUX.  The terms "serious," "unexpected,"
and "adverse drug experience" as used in this Section 9(a) shall have the same
meaning as the definitions set forth in Title 21 of the Code of Federal
Regulations, Section 314.80(a), and in effect at the time of Interneuron's
notice of report to Wyeth-Ayerst.  Interneuron shall notify Wyeth-Ayerst
immediately upon receiving notice of (1) information concerning any incident
that causes REDUX or its labeling to be mistaken for, or applied to, another
article;  or (2) information concerning any change or deterioration in REDUX. 
Any repeated failure to notify Wyeth-Ayerst under this provision shall be
grounds for termination, at Wyeth-Ayerst's option.

          (b)  Interneuron shall report monthly to Wyeth-Ayerst all other
significant information concerning any complaint of any kind regarding REDUX,
its labeling, quality or packaging, including but not limited to, any adverse
drug experience not reported pursuant to Section 9(a) above.


                                      -12-
<PAGE>

          (c)  It is understood and agreed that the reporting requirements set
forth in this Section 9 are based on Wyeth-Ayerst policies and procedures and
regulatory reporting requirements.  Accordingly, in the event of changes to
regulatory requirements or Wyeth-Ayerst policies and procedures for adverse
experience reporting, Interneuron agrees to comply with all reasonable revised
notification procedures as requested in writing by Wyeth-Ayerst.

          (d)  Wyeth-Ayerst shall evaluate all adverse drug experiences
associated with REDUX, including those reported to Wyeth-Ayerst by Interneuron,
and appropriately report serious or unexpected adverse drug experiences to FDA.

10.  RETURN/RECALL

          (a)  Any REDUX returned to Interneuron shall be shipped to Wyeth-
Ayerst's nearest facility, with any reasonable or authorized shipping or other
documented direct cost to be paid by Wyeth-Ayerst.  Interneuron shall incur no
liability of any nature in the handling of such returns.  Interneuron, at its
option, may advise the customer who made the return that the REDUX has been
returned to Wyeth-Ayerst.

          (b)  At Wyeth-Ayerst's request, Interneuron shall reasonably assist
Wyeth-Ayerst in obtaining and receiving REDUX that has been recalled, and any
direct documented costs incurred by Interneuron with respect to participating in
such recall shall be reimbursed by Wyeth-Ayerst.

11.   FORCE MAJEURE

     Neither of the parties shall be liable or be in breach of any provision of
this Agreement for any failure or omission on its part to perform any
obligations because of force majeure, including, but not limited to war, riot,
fire, explosion, flood, sabotage,


                                      -13-
<PAGE>

accident or breakdown of machinery; unavailability of fuel, labor, containers or
transportation facilities; accidents of navigation or breakdown or damages of
vessels.  or other conveyances for air, land or sea; other impediments or
hindrances to transportation; strike or other labor disturbances; governmental
regulation, or any other cause beyond the reasonable control of the party; and
provided that such failure or omission resulting from one of the above causes is
cured as soon as practicable after the occurrence of one or more of the
abovementioned causes.

12.  RECORD AND AUDIT PROVISIONS

     Each party shall keep accurate records in sufficient detail (a) Interneuron
of the number of Details and the positioning of such Details for REDUX and other
information regarding all Details for REDUX or additional Wyeth-Ayerst products
and (b) Wyeth-Ayerst of the financial computations required to make payments to
Interneuron which are required under this Agreement.  Each party, upon
reasonable advance notice to the other party, and at the requesting party's
expense, shall permit an independent, certified public accountant of the
requesting party, except one to whom the other party has reasonable objections,
to have access during ordinary business hours to the other party's records which
pertain to the performance of this Agreement.  Such examination shall not take
place more than twice a Contract Year during the term of this Agreement.  These
requests with respect to any year shall terminate two (2) years after the end of
such year.

13.  WARRANTY AND INDEMNITY


                                      -14-
<PAGE>

          (a)  Each party warrants and represents to the other that it has the
full right and authority to enter into this Agreement, and that it is not aware
of any impediment that would inhibit its ability to perform its obligations
under this Agreement.

          (b)  Wyeth-Ayerst shall indemnify, defend, and hold Interneuron
harmless from any claims, damages, actions, liabilities, losses, costs and
expenses, including reasonable attorneys' fees, (hereinafter "Claims") of a
third party, arising from the promotion of REDUX by Wyeth-Ayerst in violation of
approved labeling or any applicable statute or regulation, or any breach by
Wyeth-Ayerst of its obligations under this Agreement unless such Claims arise
from a manufacturing or design defect of REDUX, Interneuron's breach of its
responsibilities under this Agreement or a negligent or wrongful act of
Interneuron.  Interneuron shall notify Wyeth-Ayerst immediately of any such
Claims and shall cooperate with Wyeth-Ayerst in the defense of such Claims.

          (c)  Interneuron shall indemnify and hold Wyeth-Ayerst harmless from
any Claims of a third party arising out of the storage and distribution of REDUX
or Samples of REDUX by Interneuron, the Copromotion of REDUX by Interneuron in
violation of approved labeling or any applicable statute or regulation, any
breach by Interneuron of its obligations under the Agreement, a manufacturing or
design defect of REDUX, unless such Claims arise from Wyeth-Ayerst's breach of
its responsibilities under this Agreement, or a negligent or wrongful act of
Wyeth-Ayerst.  Wyeth-Ayerst shall notify Interneuron immediately of any such
claim, damage or loss and shall cooperate with Interneuron in the defense of
such Claims.

          (d)  Each party agrees to give the other prompt written notice of any
Claims made for which the other might be liable under the foregoing
indemnification



                                      -15-
<PAGE>

together with the opportunity to defend, negotiate, and settle such Claims.  The
party seeking indemnification under this Agreement shall provide the other party
with all information in its possession, authority, and assistance necessary to
enable the indemnifying party to carry on the defense of such Claims.  Neither
party shall be responsible or bound by any settlement made without its prior
written consent.

13.  WAIVER/MODIFICATION

     Any term or condition of this Agreement may be waived or modified at any
time by the party entitled to the benefit thereof by a written instrument
executed by both parties.  No delay or failure on the part of any party in
exercising any rights hereunder and no partial or single exercise thereof, will
constitute a waiver of such rights or of any rights hereunder.

14.  GOVERNING LAW

     This Agreement shall be construed and the respective rights of the parties
hereto determined according to the substantive laws of the State of New Jersey
notwithstanding the provisions to the contrary governing conflict of laws under
such law.

15.  SEVERANCE

     If any one or more of the provisions of this Agreement shall be held to be
invalid, illegal or unenforceable, the validity, legality or enforceability of
the remaining provisions hereof shall not in any way be affected or impaired
thereby.  To the extent permitted by applicable law, each party waives any
provision of law which renders any provision hereof invalid, illegal or
unenforceable in any respect.  In the event any provision of this Agreement
shall be held to be invalid, illegal or unenforceable, the parties hereto shall
negotiate in good faith to substitute a valid, legal and enforceable provision
which, insofar as practical, implements the purposes hereof.


                                      -16-
<PAGE>

16.  ENTIRE AGREEMENT

     This Agreement contains the entire agreement between the parties in respect
of the subject matter hereof and supersedes and cancels all previous agreements,
negotiations, commitments and writings between the parties hereto with respect
to the subject matter hereof and may not be changed or modified in any manner or
released, discharged, abandoned or otherwise terminated unless in writing and
signed by the duly authorized officers or representatives of the parties.

17.  NOTICE

     Any notice or request required or permitted to be given in connection with
this Agreement shall be deemed to have been sufficiently given if sent by
prepaid registered mail or telecopier to the intended recipient as set forth
below or such other business address as may have been furnished in writing by
the intended recipient to the sender.  The date of mailing or telecopying shall
be deemed to be the effective date on which notice was given, provided that all
telecopies shall have been confirmed either electronically or by the intended
recipient if such confirmation is received within twenty-four (24) hours of its
transmission.

               WYETH-AYERST
               Joseph M.  Mahady, President
               Pharmaceutical Division

               with copy to:
               Charles N.  Ross, Esq., Director
               Legal Division

               INTERNEURON
               Glenn L.  Cooper, M.D., President and Chief Executive Officer

               with copy to:
               Mark S.  Butler, Esq.,
               Executive Vice President and
                 Chief Administrative Officer


                                      -17-
<PAGE>

18.  PUBLIC ANNOUNCEMENTS

     Any public announcements or publicity with respect to this Agreement or the
transaction contemplated herein shall be at such time and in such manner as
Wyeth-Ayerst and Interneuron shall agree, provided that nothing herein shall
prevent either party, following notice and opportunity to review by the other,
from making such public announcements as such party's legal obligations require.

19.  SUPREMACY OF SUBLICENSE AGREEMENT

     Nothing in this Agreement:  shall in any way supersede, alter or diminish
any rights either of the parties may have under the Sublicense Agreement.

     IN WITNESS WHEREOF, the parties have caused this Agreement to be executed
by the duly authorized officers effective as of June 1, 1996.



                              WYETH-AYERST LABORATORIES

                              By:   /s/ Joseph M. Mahady                
                                 ---------------------------

                              Name:  Joseph M. Mahady

                              Title:  President, Pharmaceutical Division

                              INTERNEURON PHARMACEUTICALS INC.

                              By:   /s/ Mark S. Butler
                                 ---------------------------

                              Name:  Mark S. Butler

                              Title:  EVP


                                      -18-
<PAGE>

The information below marked by * and [ ] has been omitted pursuant to a request
for confidential treatment.  The omitted portion has been separately filed with
the Commission.

                                    EXHIBIT A



[






                                                            ]


                                      -19-