Document:

<PAGE>   1
                                                                    EXHIBIT 10.8

                              ASSIGNMENT AGREEMENT

                                      AMONG

                                  ZONAGEN, INC.

                                  GAMOGEN, INC.

                                       AND

                              DR. ADRIAN ZORGNIOTTI

                                 APRIL 13, 1994

<PAGE>   2

                                TABLE OF CONTENTS

<TABLE>
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                                                                                                            PAGE
<S>                                                                                                         <C>
ARTICLE I
Definitions                                                                                                   1

ARTICLE II
Assignment of Subject Technology                                                                              3
2.1      Assignment of Technology                                                                             3
2.2      Agreement Not to Compete                                                                             5
2.3      Regulatory Approval                                                                                  6
2.4      Regulatory Filings                                                                                   6

ARTICLE III
Technology Rights                                                                                             7
3.1      Infringement                                                                                         7
3.2      Notice of Other Patents                                                                              7
3.3      Trademark                                                                                            7
3.4      Improvements                                                                                         7
3.5      Patent Prosecution and Maintenance                                                                   7

ARTICLE IV
Confidentiality                                                                                               8
Confidentiality                                                                                               8

ARTICLE V
Representations and Warranties                                                                                8
5.1      Representations and Warranties of Gamogen                                                            8
5.2      Representations and Warranties of Zonagen                                                            9

ARTICLE VI
Conditions to Closing                                                                                        10
6.1      Zonagen's Conditions to Closing                                                                     10
6.2      Gamogen's Conditions to Closing                                                                     11
6.3      Zorgniotti's Conditions to Closing                                                                  11

ARTICLE VII
Miscellaneous                                                                                                11
7.1      Notices                                                                                             11
7.2      Amendments                                                                                          12
7.3      Waiver                                                                                              12
7.4      Headings                                                                                            12
7.5      Applicable Law                                                                                      12
7.6      Severability                                                                                        12
7.7      Further Assurances                                                                                  13
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                                TABLE OF CONTENTS
                                   (Continued)
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                                                                                                            PAGE
<S>                                                                                                         <C>
7.8      Negation of Agency                                                                                  13
7.9      Publicity                                                                                           13
7.10     Registration and Filing of the Agreement                                                            13
7.11     Entire Agreement                                                                                    13
7.12     Beneficiaries                                                                                       13
7.13     Affiliates of Parties                                                                               14
7.14     Specific Enforcement                                                                                14
7.15     Dispute Resolution                                                                                  14
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<PAGE>   4

                              ASSIGNMENT AGREEMENT

         This Assignment Agreement (the "Agreement") is entered into as of April
13, 1994 among Zonagen, Inc., a Delaware corporation ("Zonagen"), Gamogen, Inc.,
a New York corporation ("Gamogen"), and Dr. Adrian Zorgniotti ("Zorgniotti").

         WHEREAS, Zorgniotti has made certain inventions and improvements in
using phentolamine-based compounds for the modification of human sexual
response; and

         WHEREAS, Zorgniotti has assigned all of his rights to such inventions
and improvements to Gamogen; and

         WHEREAS, Zonagen and Gamogen executed a letter of intent dated December
8, 1993 (the "Letter of Intent") relating to the potential acquisition by
Zonagen of Gamogen's rights to such inventions; and

         WHEREAS, Zonagen has successfully completed its due diligence review
and proof of principal study with respect to such product as contemplated by the
Letter of Intent; and

         WHEREAS, the parties hereto desire to enter into a definitive agreement
for the purchase by Zonagen and sale by Gamogen and Zorgniotti of its and his
rights in such inventions under the terms and conditions set forth in the Letter
of Intent;

         NOW, THEREFORE, in consideration of the foregoing and the mutual
promises and covenants contained in this Agreement, the parties hereto agree as
follows:

                                    ARTICLE I
                                   DEFINITIONS

         The following capitalized terms shall have the following meanings.

         "Affiliate" means with respect to any Person, any other Person which
directly or indirectly, by itself or through one or more intermediaries,
controls, or is controlled by, or is under direct or indirect common control
with, such Person. The term "control" means the possession, direct or indirect,
of the power to direct or cause the direction of the management and policies of
a Person, whether through the ownership of voting securities, by contract or
otherwise.

         "Competing Product" means any phentolamine-based product for the
modification of human sexual response other than a phentolamine-papaverine
injectable product. No vacuum pump or other device for the treatment of male
impotence shall be a Competing Product under this Agreement.

         "Confidential Information" means all proprietary information reduced to
writing and labeled "Confidential" acquired by one Party from another Party
under this Agreement, including, but not limited to, present techniques,
inventions, practices, knowledge, know-how,

                                      -1-
<PAGE>   5

conception, skill, experience, test data, including pharmacological and clinical
test data, analytical and quality control data and other proprietary information
and trade secrets; provided, however, that the Subject Technology assigned to
Zonagen under this Agreement shall not be Confidential Information in Zonagen's
possession.

         "Effective Date" means the date of the execution of this Agreement by
all the parties hereto.

         "FDA" means the United States Food and Drug Administration of the
Department of Health and Human Services and any successor entities. References
herein to FDA shall include, to the extent applicable, any comparable Foreign
Regulatory Authority.

         "Foreign Filing" means any application or regulatory filing to be filed
hereunder with a Foreign Regulatory Authority for approval to manufacture or
sell a Royalty-Bearing Product outside the United States.

         "Foreign Regulatory Authority" means any foreign counterpart of the FDA
or other similar foreign health agency or authority.

         "Net Sales" means the gross sales of a Royalty-Bearing Product
hereunder as reflected in invoices to Third Parties, less any applicable taxes
or duties, and any reasonable rebates or allowances (including but not limited
to rebates to any Third Party), and payments to distributors or mail order
houses, chargebacks, shipping or freight charges prepaid or allowed, and less
the value of returned trade goods and reasonable trade cash discounts actually
given.

         "Patent" means any United States or foreign letters patent issuing on
U. S. Patent Application N9 08/106,434, filed August 13, 1993 by Zorgniotti and
Gioco, including any continuation, continuation-in-part, division or any
substitute application, any patent issued with respect to such Patent
Application, any reissue, reexamination, renewal or extension of any such
patent, and any confirmation patent or registration patent or patent of addition
based on any such patent, and all foreign counterparts of any of the foregoing.

         "Person" means any individual, corporation, partnership, limited
liability company, joint venture or other legal business entity.

         "Royalty-Bearing Product" means any phentolamine-based formulation,
composition or vehicle for administration in the modification of human sexual
response or improvements developed under this Agreement.

         "Subject Technology" means all Technology relating to the invention or
the development of the invention owned or controlled by Gamogen or Zorgniotti as
of the date hereof, including the Patent, any biologic master file, drug master
file, investigational new drug application, new drug application and
Improvements developed by Zorgniotti pursuant to this Agreement.

                                      -2-
<PAGE>   6

         "Technology" means any (i) ideas, methods, inventions, patents, patent
applications, techniques, processes, know-how, trade secrets, Confidential
Information and other information and (ii) compounds, formulations, and other
natural and man-made materials.

         "Territory" means all countries of the world.

         "Third Party" means any entity other than Zonagen, Gamogen, Zorgniotti
or their affiliates.

                                   ARTICLE II
                        ASSIGNMENT OF SUBJECT TECHNOLOGY

         2.1      ASSIGNMENT OF TECHNOLOGY.

         (a) Effective as of the Effective Date, Gamogen and Zorgniotti each
hereby assigns, transfers and conveys to Zonagen all of its right, title and
interest in the Subject Technology, including the right of Zonagen to file in
its name applications for patents and similar protection for the Subject
Technology in a country or countries foreign to the United States, all
international rights of priority associated with the Subject Technology, and all
technical information, improvements and know-how, whether or not patented,
pertaining to the Subject Technology, except that Gamogen's and Zorgniotti's
rights, if any, to a two-drug injectable product using phentolamine-papaverine
for the treatment of male impotency shall remain vested in Gamogen and
Zorgniotti and are not assigned hereunder. Consistent with the terms of this
Section 2.1(a), each of Gamogen and Zorgniotti shall, on execution of this
Agreement, also execute an assignment in the form attached hereto as Exhibit A.

         (b) As consideration of the assignment to Zonagen of the Subject
Technology by Gamogen pursuant to Section 2.1(a), Zonagen shall deliver to
Gamogen the following:

                  (i) $50,000 upon the execution of this Agreement;

                  (ii) $50,000 upon delivery satisfactory to Zonagen of all
         documentation in the possession of Gamogen and Zorgniotti relating to
         the Subject Technology; and

                  (iii) A royalty equal to six percent of the first $100 million
         of Net Sales, five percent of Net Sales in excess of $100 million and
         less than $200 million, four percent of Net Sales in excess of $200
         million and less than $300 million, and three percent of Net Sales in
         excess of $300 million, in each case payable quarterly. Such royalty
         payments shall be reduced by two percentage points with respect to Net
         Sales in any country in which (A) patent protection for the
         Royalty-Bearing Product is not available under the laws of such
         country, (B) Zonagen determines that patent protection under the laws
         of such country would be ineffective to secure exclusive rights to sell
         the Royalty-Bearing Product in such country or that patent protection
         under the laws of such country cannot be obtained despite Zonagen's
         best efforts or (C) patent protection for the Royalty- Bearing Product
         has expired in such country. The royalties payable hereunder shall be
         computed on aggregate Net Sales, and not annual Net Sales. On sales
         within each country

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         in the Territory, such royalties shall be payable for the greater of
         ten years following the date Zonagen initiates reasonable marketing
         efforts in such country or the time during which Zonagen has a valid
         patent for the Royalty-Bearing Product in any country within the
         Territory. Zonagen shall keep and maintain complete and accurate books
         in respect of its activities under this Agreement for which royalty
         payments may be required, in accordance with applicable accounting
         principles consistently applied. Upon 48 hours' prior notice to
         Zonagen, Gamogen may inspect such records at its own expense to verify
         the accuracy of royalty payments made to Gamogen hereunder.

         (c) In consideration of the assignment of the Subject Technology by
Zorgniotti pursuant to Section 2.1(a), Zonagen shall sponsor, for Zonagen's sole
benefit, research in the area of male impotence, with the goals of such research
efforts to be agreed upon from time to time by Zonagen and Zorgniotti. Zonagen
will provide up to $100,000 per year for three years for funding of research in
the area of male impotency, such funds to be used for such research efforts as
Zonagen, in its sole discretion, may determine. Zorgniotti shall be appointed as
Zonagen's Chief Scientific Advisor--Male Impotency and will receive $25,000 per
year for two years, payable in equal monthly installments, for serving in such
capacity. Zonagen may terminate funding, as contemplated above, of its male
impotency research projects for any business related reason, but, except as
provided below, Zonagen's obligation to pay Zorgniotti $25,000 per year for
acting as Zonagen's Chief Scientific Advisor--Male Impotency may not be canceled
until the expiration of two years following the Effective Date. If Zonagen
terminates project funding, then Zorgniotti may resign as Chief Scientific
Advisor--Male Impotency and may continue such research independent of Zonagen,
subject to Zonagen's Option as set forth in Section 2.1(d). If Zorgniotti
resigns, Zonagen's obligation to pay Zorgniotti $25,000 per year shall
terminate. Zorgniotti may not assign his rights or obligations as Chief
Scientific Advisor - Male Impotency without Zonagen's written consent.

         (d) Gamogen and Zorgniotti each hereby grant to Zonagen a right of
first negotiation (the "Option") to purchase or exclusively license any
improvement to any Royalty-Bearing Product, Competing Product or other product
(other than any two drug injectable product using phentolamine-papaverine)
relating to the modification of human sexual response developed by Gamogen or
Zorgniotti (collectively, an "Option Product"). Neither Gamogen nor Zorgniotti
(the "Offering Party") shall offer to sell or offer to grant an exclusive
license (an "Offer") on any Option Product developed by Gamogen or Zorgniotti,
as the case may be, before delivering written notice to Zonagen (the "Zonagen
Offer Notice") which shall disclose the terms of the Offer. Zonagen shall have
an Option to purchase all rights to any Option Product at the price and on the
terms specified in the Zonagen Offer Notice. Zonagen shall deliver to the
Offering Party as soon as practicable, but in any event within 60 days after the
receipt of the Zonagen Offer Notice (the "Notice Period"), a notice (the
"Zonagen Election Notice") stating whether Zonagen has elected to purchase the
rights to such Option Product. If Zonagen has elected to purchase such Option
Product from the Offering Party, the transfer of the rights to such Option
Product shall be consummated as soon as practicable after the delivery of the
Zonagen Election Notice. If Zonagen elects not to purchase such Option Product,
the Offering Party, within 180 days after expiration of the Election Period, may
transfer such Option Product to a Third Party at a price not less than the price
specified in the Zonagen Offer Notice and on other terms no more favorable to
such Third Party than offered to Zonagen. If the closing of such transaction
does not

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occur within such 180-day period, then the obligation of Gamogen or
Zorgniotti, as the case may be, to give Zonagen notice of a proposed Offer under
the terms and conditions set forth above shall once again become effective.

         2.2      AGREEMENT NOT TO COMPETE.

         (a) Neither Gamogen nor its Affiliates shall, during the time this
Agreement is in force, but in no event longer than the time during which Zonagen
is paying royalties to Gamogen under this Agreement, nor shall Zorgniotti or any
of his Affiliates during the term he is acting as Zonagen's Chief Scientific
Advisor--Male Impotency under this Agreement, directly or indirectly, engage or
invest in, finance, own, manage, operate, control, or participate in the
ownership, management, operation or control of, be employed by, associated or in
any manner connected with, render services or advice to, or assist (whether or
not for compensation), any person or entity whose products constitute, in whole
or in part, a Competing Product (a "Competing Business"); provided, however,
that this Section 2.2(a) shall not preclude Gamogen or its Affiliates or
Zorgniotti and his Affiliates from (i) selling vacuum pumps or any other device
for the treatment of male impotency or (ii) investing in up to an aggregate 1%
of the securities of any enterprise engaged in a Competing Business (but without
otherwise participating in the activities of such enterprise) if such securities
are listed on any United States national securities exchange or have been
registered under Section 12(g) of the Securities Exchange Act of 1934, provided
that Gamogen and its Affiliates combined do not purchase or hold (directly or
indirectly) an aggregate equity interest of more than 5% in any such enterprise.

         (b) As consideration for Gamogen's agreement not to compete set forth
in Section 2.2(a), Zonagen shall deliver to Gamogen $200,000 of the common
stock, par value $.001 per share, of Zonagen ("Common Stock") on the day Zonagen
receives FDA approval for its completed Phase II clinical trials with respect to
a Royalty-Bearing Product, based on the closing sale price for the Common Stock
on the principal market on which the Common Stock is traded on such day;
provided, however, that if Zonagen has not received FDA approval for such
completed Phase II clinical trials by April 13, 1996, then Zonagen must, within
30 days, either (A) transfer all rights to the Subject Technology, including any
Royalty-Bearing Products, back to Gamogen or (B) deliver to Gamogen $200,000 of
Common Stock based on the closing sale price for the Common Stock on the
principal market on which the Common Stock is traded on April 13, 1996. The
certificates evidencing the Common Stock to be issued pursuant to this Section
2.2(b) will bear the following legend:

                  The securities evidenced hereby have not been registered under
                  the Securities Act of 1933, as amended (the "Act"), and may
                  not be transferred (other than pursuant to Rule 144 or any
                  similar or analogous rule or rules) except pursuant to an
                  effective registration under the Act or in a transaction
                  which, in the opinion of counsel reasonably satisfactory to
                  the Company, qualifies as an exempt transaction under the Act
                  and the rules and regulations promulgated thereunder.

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         (c) As consideration for Zorgniotti's agreement not to compete set
forth in Section 2.2(a), Zonagen hereby agrees to extend Zorgniotti's term as
Chief Scientific Advisor--Male Impotency for one year following the expiration
of two years from the Effective Date under the same terms and conditions set
forth in Section 2.1(c).

         2.3      REGULATORY APPROVAL.

         (a) Zonagen shall use commercially reasonable efforts to obtain
necessary regulatory approval, and following approval to initiate marketing of,
any Royalty-Bearing Product in the Territory where it is commercially feasible
to market such Royalty-Bearing Product. Such efforts in each country within the
Territory shall be conducted pursuant to a time schedule to be agreed upon by
Zonagen and Gamogen. Zonagen shall be responsible for all costs associated with
obtaining such approvals.

         (b) If Zonagen determines not to seek regulatory approval for a sale of
the Royalty-Bearing Product in a specific country within the Territory, and
Gamogen desires to sell such Royalty-Bearing Product in such country, then upon
Gamogen's request and at Zonagen's cost, Zonagen will either file for such
regulatory approval or, if such approval has been obtained, initiate reasonable
marketing efforts, within 120 days of such request, or Zonagen shall assign the
exclusive rights to Gamogen to sell the Royalty-Bearing Product in such country,
and Gamogen may proceed, at Gamogen's expense, to obtain approval or, if such
approval has been obtained, initiate reasonable marketing efforts, to market the
Royalty-Bearing Product in such country. Gamogen shall pay Zonagen a royalty
equal to two percent of any Net Sales of the Royalty-Bearing Product sold by
Gamogen in each such country.

         (c) If Zonagen fails to meet any deadline imposed upon Zonagen by the
regulatory approval time schedule established pursuant to Section 2.2(a)
relating to obtaining regulatory approval in a particular country, and such
failure is not the result of a delay caused by any Third Party, the FDA or other
governmental regulatory agency, then Gamogen may send Zonagen a written notice
notifying Zonagen that Zonagen must commence within 120 days the process of
obtaining such regulatory approval. If Zonagen does not commence such activities
within such 120 day period, then Gamogen may proceed to obtain such regulatory
approvals at its own expense, and upon Gamogen's receipt of such approvals,
Gamogen shall have the right to sell any Royalty-Bearing Product in such
country. If Zonagen determines not to seek regulatory approval for sale of a
Royalty-Bearing Product in the United States, and Gamogen elects to seek such
approval at its expense under the terms of this Section 2.3(b), then Gamogen and
Zonagen shall negotiate a royalty to be payable to Zonagen by Gamogen with
respect to Gamogen's sales of such Royalty-Bearing Product in the United States,
such royalty to be in an amount based on the relative funds spent by Zonagen and
Gamogen in connection with obtaining such regulatory approval in the United
States, but in no event greater than 3% of Gamogen's Net Sales.

         2.4      REGULATORY FILINGS.

         Gamogen and Zorgniotti shall cooperate with Zonagen in making all
regulatory filings with the FDA and all Foreign Filings, all of which shall be
in Zonagen's name.

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<PAGE>   10

                                   ARTICLE III
                                TECHNOLOGY RIGHTS

         3.1      INFRINGEMENT.

         (a) Zonagen shall defend or prosecute, at its sole cost and expense,
any and all claims of infringement that may be brought in connection with the
Patents. Zonagen, Gamogen and Zorgniotti shall immediately notify each other in
writing of any activity of any type or nature whatsoever which they believe to
be an infringement of the Patents. Gamogen and Zorgniotti shall cooperate with
Zonagen in all matters concerning any such infringement, and shall make
available any relevant records, documents, information, evidence, and the like,
if requested to do so by Zonagen.

         3.2      NOTICE OF OTHER PATENTS.

         Each Party shall bring to the attention of the other Party any patent
or patent application it discovers, or has discovered, and which relates to the
subject matter of this Agreement, and shall cooperate with each other so that
each Party can determine whether valid rights of a Third Party may be infringed.

         3.3      TRADEMARK.

         Zonagen shall apply for and own and bear the expense of maintaining a
trademark for any Royalty-Bearing Products.

         3.4      IMPROVEMENTS

         Improvements to the Subject Technology relating to the modification of
human sexual response, whether patentable or not, conceived or made by
Zorgniotti, whether pursuant to the research funding provided by Zonagen
pursuant to Section 2.1(c) or otherwise, shall be Zonagen's property. Any
improvements developed by Zorgniotti after the expiration of the research
agreement set forth in Section 2.1(c) that are not based on information obtained
or developed in the course of such research shall be owned by Zorgniotti. Any
improvements developed by Gamogen that are not based on the Subject Technology
shall belong to Gamogen. Any other improvements developed through the joint
efforts of the parties hereto shall be owned jointly by the parties hereto based
on their relative participation and funding of the development of such
Improvements; provided, however, that Zonagen shall have a right of first
refusal to purchase or exclusively license Gamogen's rights in such
jointly-owned improvements.

         3.5      PATENT PROSECUTION AND MAINTENANCE.

          Zonagen shall be responsible for filing, prosecuting and maintaining
the Patent in the Territory and shall bear all costs associated therewith.
Gamogen shall have the right at its expense to file, prosecute and maintain
patents in the Territory on Improvements owned solely by Gamogen. In the case of
any such filing by Gamogen, Zonagen shall have the right to review and comment
on such filings and all patent office communications related thereto.

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<PAGE>   11

Notwithstanding the foregoing, neither Zonagen nor Gamogen assumes liability to
the other for the successful prosecution of any patent application.

                                   ARTICLE IV
                                 CONFIDENTIALITY

         4.1      CONFIDENTIALITY.

         The Parties hereto acknowledge that Confidential Information shall be
considered to be confidential by them, and that they shall not disclose to
others (including to any Affiliates not bound by like conditions of
confidentiality), nor make any use of the Confidential Information received from
another Party for any purpose other than as contemplated in this Agreement,
without the prior written consent of the Party who delivered such Confidential
Information before five years after first disclosure thereof, except to the
extent any of the Confidential Information (i) was known to such Party before
the disclosure hereunder or developed independent of the disclosure; (ii) is or
becomes publicly known through no fault or omission attributable to such Party
or (iii) is rightfully given to such Party from sources independent of the Party
who delivered such Confidential Information which sources rightfully possess
such information. Upon notice to the other Parties, a Party also may disclose
Confidential Information to government authorities necessary to meet applicable
regulations.

                                    ARTICLE V
                         REPRESENTATIONS AND WARRANTIES

         5.1 REPRESENTATIONS AND WARRANTIES OF GAMOGEN. Gamogen represents and
warrants to Zonagen as follows:

         (a) Gamogen is a corporation duly incorporated, validly existing and in
good standing under the laws of the State of New York and has all requisite
corporate power and authority to own and operate its properties and assets and
to carry on its business as it is presently being conducted and as proposed to
be conducted.

         (b) This Agreement has been duly executed and delivered by Gamogen and
Zorgniotti and constitutes the valid and binding obligation of Gamogen and
Zorgniotti, enforceable against Gamogen and Zorgniotti in accordance with its
terms, except as enforceability may be limited by bankruptcy, fraudulent
conveyance, insolvency, reorganization, moratorium and other laws relating to
creditors' rights generally and by general equitable principles.

         (c) Gamogen is not in violation or default of any provisions of its
Certificate of Incorporation or Bylaws.

         (d) The execution, delivery and performance of this Agreement will not
result in any violation of, be in conflict with, or constitute a default under,
with or without the passage of time or the giving of notice, any material
contract, obligation or commitment to which Gamogen is a

                                      -8-
<PAGE>   12

party or by which it is bound or the terms and conditions of Gamogen's
Certificate of Incorporation or By-Laws.

         (e) There are no consents or waivers, other than those which have been
obtained, which Gamogen must obtain so as to be able to fulfill its obligations
and to provide Zonagen with all its rights under this Agreement.

         (f) Gamogen is acquiring Common Stock to be issued pursuant to Section
2.2(b) for its own account for investment and not with a view towards the
resale, transfer or distribution thereof, nor with any present intention of
distributing such Common Stock.

         (g) Gamogen understands that the Common Stock to be issued to it
hereunder will not be registered under the Securities Act of 1933, as amended
(the "Act"), nor qualified under any state securities laws, and that such Common
Stock will be sold pursuant to an exemption from such registration and
qualification, based in part upon the representations of Gamogen contained
herein. Gamogen will not offer to sell or otherwise dispose of the Common Stock
so acquired by it in violation of the registration requirements of the Act or
any applicable state securities laws.

         (h) Gamogen has such knowledge and experience in financial and business
matters that it is capable of evaluating the merits and risks of the investment
contemplated by this Agreement and Gamogen is able to bear the economic risk of
its investment in Zonagen.

         (i) Gamogen owns all right, title and interest in the Subject
Technology, free and clear of any liens, encumbrances, pledges and claims of any
Third Party, and the assignment of the Subject Technology to Zonagen hereunder
does not violate any right of any Third Party.

         (j) Except for certain travel costs to be paid to Gamogen by Zonagen
which are listed on Schedule 5.1(j) and are to be paid within 30 days following
execution of this Agreement, Gamogen has been reimbursed all expenses required
to be paid by Zonagen, and Zonagen has met all of its other obligations to
Gamogen under the Letter of Intent to Gamogen's complete satisfaction.

         5.2      REPRESENTATIONS AND WARRANTIES OF ZONAGEN

         (a) Zonagen is a corporation duly incorporated, validly existing and in
good standing under the laws of the State of Delaware and has all requisite
corporate power and authority to own and operate its properties and assets and
to carry on its business as it is presently being conducted.

         (b) The authorized capital stock of Zonagen consists of: (i) 20,000,000
shares of Common Stock, par value $0.001 per share, of which 3,667,536 shares
are duly and validly issued, fully paid, nonassessable and outstanding; and (ii)
5,000,000 shares of Preferred Stock, par value $0.001 per share, of which none
are outstanding.

                                      -9-
<PAGE>   13

         (c) This Agreement has been duly executed and delivered by Zonagen and
constitutes the valid and binding obligation of Zonagen, enforceable against
Zonagen in accordance with its terms, except as enforceability may be limited by
bankruptcy, fraudulent conveyance, insolvency, reorganization, moratorium and
other laws relating to creditors' rights generally and to general equitable
principles. No other corporate action is necessary to authorize the execution or
delivery of this Agreement or the performance by Zonagen of its obligations
hereunder.

         (d) The shares of Common Stock to be issued pursuant to this Agreement,
when delivered for the consideration specified herein, will be duly authorized,
validly issued, fully paid and nonassessable shares of Zonagen.

         (e) Neither the nature of the business which Zonagen currently conducts
or proposes to conduct, nor any relationship between Zonagen and any other
person, nor any circumstance in connection with the creation, authorization,
issuance, offer or sale of the Common Stock is such as to require a consent,
approval or authorization of, or filing, registration or qualification with, any
governmental authority on the part of Zonagen or the vote, consent or approval
in any manner of the holders of any security of Zonagen as a condition to the
execution, delivery and performance of this Agreement.

         (f) Zonagen has filed all material documents necessary to qualify to do
business as a foreign corporation in, and Zonagen is in good standing under, the
laws of each jurisdiction in which the conduct of Zonagen's business or the
nature of the property owned by Zonagen requires such qualification, except
where the failure so to qualify would not have a material adverse affect on the
business or financial position of Zonagen.

         (g) Zonagen is not in violation or default of any provisions of its
Certificate of Incorporation or Bylaws.

         (h) The execution, delivery and performance of this Agreement will not
result in any violation of, be in conflict with, or constitute a default under,
with or without the passage of time or the giving of notice, any material
contact, obligation or commitment to which Zonagen is a party or by which it is
bound or the terms and conditions of Zonagen's Certificate of Incorporation or
By-Laws.

         (i) There are no consents or waivers, other than those which have been
obtained, which Zonagen must obtain so as to be able to fulfill its obligations
to Gamogen under this Agreement.

                                   ARTICLE VI
                              CONDITIONS TO CLOSING

         6.1      ZONAGEN'S CONDITIONS TO CLOSING.

         Zonagen shall have received an opinion of counsel to Gamogen, in form
and substance satisfactory to Zonagen and its counsel, as to the matters set
forth in Sections 5.1(a), (b), (c), (d), (e) and (i).

                                      -10-
<PAGE>   14

         6.2      GAMOGEN'S CONDITIONS TO CLOSING.

         Gamogen shall have received an opinion of counsel to Zonagen, in form
and substance satisfactory to Gamogen and its counsel, as to the matters set
forth in Section 5.2.

         6.3      ZORGNIOTTI'S CONDITIONS TO CLOSING.

         Zorgniotti shall have received an opinion of counsel to Zonagen, in
form and substance satisfactory to Zorgniotti and his counsel, as to the matters
set forth in Sections 5.2(a), (b), (c), (f), (g), (h) and (i).

                                   ARTICLE VII
                                  MISCELLANEOUS

         7.1      NOTICES.

         Any notice or other communication required or permitted to be given by
any party under this Agreement shall be effective when delivered, if delivered
by hand or by electronic facsimile or five days after mailing if mailed by
registered or certified mail, postage prepaid and return receipt requested, and
shall be addressed to each Party at the following addresses or such other
address as may be designated by notice pursuant to this Section:

         IF TO ZONAGEN:

                  Zonagen, Inc.
                  2408 Timberloch Place, B-4
                  The Woodlands, Texas 77380
                  Attn: President
                  Facsimile: (713) 363-8796

         WITH COPY TO:

                  Porter & Hedges, L.L.P.
                  700 Louisiana, Suite 3500
                  Houston, Texas 77002
                  Attn: Samuel N. Allen
                  Facsimile: (713) 228-1331

         IF TO GAMOGEN:

                  Gamogen, Inc.
                  17 Industrial Place
                  Middleton, New York 10940
                  Attention: President
                  Facsimile: (914) 344-4037

                                      -11-
<PAGE>   15
         WITH COPY TO:

                  Steven Feinberg
                  Solon, Marrow & Dykman
                  685 Third Avenue
                  New York, New York 10017
                  Facsimile: (212) 661-3339

         IF TO ZORGNIOTTI:

                  Dr. Adrian Zorgniotti
                  33 East 74th Street
                  New York, New York 10021

         7.2      AMENDMENTS.

         No amendment, modification or addition hereto shall be effective or
binding on either Party unless set forth in writing and executed by the Party
against whom enforcement is being sought.

         7.3      WAIVER.

         No waiver of any rights under this Agreement shall be deemed effective
unless contained in writing signed by the Party charged with such waiver, and no
waiver of any breach or failure to perform shall be deemed a waiver of any
future breach or failure to perform or any other right arising under this
Agreement.

         7.4      HEADINGS.

         The section headings contained in this Agreement are included for
convenience only and form no part of the agreement between the parties.

         7.5      APPLICABLE LAW.

         This Agreement shall be governed by, subject to and construed in
accordance with the laws of the State of Texas and the parties consent to the
jurisdiction of the State and Federal Courts of Texas.

         7.6      SEVERABILITY.

         If any provision of this Agreement is held to be invalid, void or
unenforceable for any reason, it shall be adjusted, if possible, rather than
voided in order to achieve the intent of the parties to the maximum extent
possible. In any event, all other provisions of this Agreement shall be deemed
valid and enforceable to the fullest extent possible.

                                      -12-
<PAGE>   16

         7.7      FURTHER ASSURANCES.

         Each party agrees to execute, acknowledge and deliver such further
instruments, and to do all such other acts as may be necessary or appropriate in
order to carry out the purposes and intent of this Agreement.

         7.8      NEGATION OF AGENCY.

         Nothing herein contained shall be deemed to create an agency, joint
venture, amalgamation, partnership, or similar relationship between Zonagen and
Gamogen.

         7.9      PUBLICITY.

         No public announcement concerning the existence or the terms of this
Agreement shall be made, either directly or indirectly, by Zonagen, Gamogen or
Zorgniotti, except as may be legally required by applicable laws, regulations,
or judicial order, without first obtaining the approval of both Zonagen and
Gamogen and agreement upon the nature, text, and timing of such announcement,
which approval and agreement shall not be unreasonably withheld. The Party
desiring to make any such public announcement shall provide the other Parties
with a written copy of the proposed announcement in sufficient time before
public release to allow such other Parties to comment upon such announcement,
before public release.

         7.10     REGISTRATION AND FILING OF THE AGREEMENT.

         To the extent, if any, that Zonagen concludes in good faith that it is
required to file or register this Agreement or a notification thereof with any
governmental authority, including without limitation the Securities and Exchange
Commission and the Competition Directorate of the Commission of the European
Communities, in accordance with applicable laws and regulations, Zonagen may do
so, and the other Parties shall cooperate in such filing or notification and
shall execute all documents reasonably required in connection therewith. The
parties shall promptly inform each other as to the activities or inquiries of
any such governmental authority relating to this Agreement, and shall cooperate
to respond to any request for further information therefrom.

         7.11     ENTIRE AGREEMENT.

         This Agreement contains the entire agreement between the Parties with
respect to the subject matter hereof. Any prior agreement, arrangement or
undertaking, whether oral or in writing, is hereby superseded.

         7.12     BENEFICIARIES.

         No person, other than Zonagen, Gamogen or Zorgniotti and their
permitted assignees hereunder, shall be deemed an intended beneficiary hereunder
or have any right to enforce any obligation of this Agreement.

                                      -13-
<PAGE>   17

         7.13     AFFILIATES OF PARTIES.

         Each Party may perform its obligations hereunder personally or through
one or more Affiliates. Neither Party shall permit any of its Affiliates to
commit any act (including any act of omission) which such Party is prohibited
hereunder from committing directly.

         7.14     SPECIFIC ENFORCEMENT.

         Each Party hereto shall be entitled to an injunction or injunctions to
prevent material breaches of this Agreement and to specifically enforce the
provisions of this Agreement, in addition to any other remedy to which such
Party may be entitled, at law or in equity.

         7.15     DISPUTE RESOLUTION.

         Each Party agrees that where (a) a Party files suit against the other
party with respect to any issue arising under this Agreement and (b) that Party
is unsuccessful in its claim, that Party shall pay all legal costs, including
reasonable attorneys' fees, of the other Party.

         IN WITNESS WHEREOF, the parties have caused this Agreement to be
executed by their duly authorized representatives as of the Effective Date.

                                        ZONAGEN, INC.

                                        By:  /s/ Joseph S. Podolski
                                            ------------------------------------

                                        GAMOGEN, INC.

                                        By:  /s/ Jesse A. Garringer
                                            ------------------------------------

                                        DR. ADRIAN ZORGNIOTTI

                                        By:  /s/ Andrew I. Seafon
                                            ------------------------------------
                                            Andrew I. Seafon, Attorney in-Fact

                                      -14-<PAGE>   1
                                                                   EXHIBIT 10.11

                          REGISTRATION RIGHTS AGREEMENT

         THIS AGREEMENT is made as of October 13th, 1994, between Zonagen, Inc.,
a Delaware corporation (the "Company") and J. Tyler Dean (the "Shareholder").

         The Shareholder is a party to a Stock Exchange Agreement of even date
herewith (the "Exchange Agreement") pursuant to which the Shareholder has sold
10,000 shares of the common stock, without par value, of Fertility Technologies,
Inc., a Massachusetts corporation, in exchange for, among other things, a number
of shares of the common stock, par value $.001 per share (the "Zonagen Common
Stock") of Zonagen, Inc., a Delaware corporation (the "Company") issuable as the
Final Payment and in satisfaction of the Remaining Shareholder Debt (each as
defined in the Exchange Agreement). To induce the Shareholder to enter into the
Exchange Agreement, the Company has agreed to provide the registration rights
set forth in this Agreement. The execution and delivery of this Agreement by the
Company is a condition to the Closing under the Exchange Agreement. Unless
otherwise provided in this Agreement, capitalized terms used and not otherwise
defined herein have the meanings set forth in Section 8 hereof.

         The parties hereto agree as follows:

         1.       DEMAND REGISTRATION.

         (a) REQUESTS FOR REGISTRATION. On one occasion after the Final Closing
Date, the Shareholder may request registration under the Securities Act of 1933,
as amended (the "Securities Act") of all or part of his Registerable Securities.
Registration requested pursuant to this Section l(a) is referred to herein as a
"Demand Registration." A request for a Demand Registration shall specify the
number of Registerable Securities requested to be registered.

         (b) AGREEMENT TO REGISTER RESALES. Zonagen agrees that promptly, but in
no event later than 30 days following the Closing Date, it will file with the
Securities and Exchange Commission (the "Commission") on Form S-3, a shelf
registration statement pursuant to Rule 415 of the Securities Act (the
"Registration Statement") covering the resale by the shareholder of all the
shares of the Zonagen Common Stock issued pursuant to the Initial Payment, and
will use its best efforts to cause the Registration Statement to be declared
effective promptly by the Commission.

         (c) RESTRICTIONS ON DEMAND REGISTRATIONS. The Company will not be
obligated to effect any Registration Demand if Form S-3 or a similar short form
("Short Form Registration") is unavailable to the Company unless the Shareholder
pays all of the Company's legal fees, as incurred' with respect to such
registration, as provided in Section 5(b).

<PAGE>   2

         2.       PIGGYBACK REGISTRATIONS.

         (a) RIGHT TO PIGGYBACK. If the Company proposes to register a primary
offering of Zonagen Common Stock under the Securities Act and the registration
form to be used may be used for the registration of Registerable Securities, the
Company will give prompt written notice to the Shareholder of its intention to
effect such a registration and will include in such registration all
Registerable Securities with respect to which the Company has received written
requests for inclusion therein within 15 days after the receipt of the Company's
notice.

         (b) RESTRICTIONS ON PIGGYBACK REGISTRATIONS. The Company will not be
obligated to effect a Piggyback Registration if (i) the registration form to be
used does not permit the registration of the Registerable Securities or (ii) the
Company is subject to a pre-existing contractual prohibition against registering
such Registerable Securities and the Company is unable to obtain a waiver of
such contractual restriction.

         (c) SELECTION OF UNDERWRITERS. If any Piggyback Registration is an
underwritten offering, the selection of investment banker and manager for the
offering shall be made by the Company.

         3.       HOLDBACK AGREEMENT.

         The Shareholder agrees not to effect any public sale or distribution
(including sales pursuant to Rule 144) of equity securities of the Company,
during the seven days before and the 90 day period beginning on the effective
date of any underwritten Piggyback Registration in which Registerable Securities
are included (except as part of such underwritten registration), unless the
underwriters managing the registered public offering otherwise agree. In
addition, the Shareholder shall be bound by any lock-up, standstill or similar
agreement that is executed by members of Zonagen's senior management in
connection with any public offering or private placement of Zonagen's
securities, provided however, that any such lock-up, standstill or similar
agreement shall not effect the Shareholder's right to sell up to 15% of his
Zonagen Common Stock per quarter pursuant to Section 6.2(b) of the Exchange
Agreement.

         4. REGISTRATION PROCEDURES. (a) Whenever the Shareholder has requested
that any Registerable Securities be registered pursuant to this Agreement, the
Company will use its best efforts to effect the registration of such
Registerable Securities in accordance with the intended method of disposition
thereof, and pursuant thereto the Company will as expeditiously as possible:

         (i) prepare and file with the Commission a registration statement with
respect to such Registerable Securities and use its best efforts to cause such
registration statement to become effective (provided that before filing a
registration statement or prospectus or any amendments or

                                      -2-
<PAGE>   3

supplements thereto, the Company will furnish to counsel selected by the
Shareholder copies of all such documents proposed to be filed, which documents
will be subject to the review and comment of such counsel);

         (ii) prepare and file with the Commission such amendments and
supplements to such registration statement and the prospectus used in connection
therewith as may be necessary to keep such registration statement effective for
a period of not less than six months and comply with the provisions of the
Securities Act with respect to the disposition of all securities covered by such
registration statement during such period in accordance with the intended
methods of disposition thereof;

         (iii) furnish to the Shareholder such number of copies of such
registration statement, each amendment and supplement thereto, the prospectus
included in such registration statement (including each preliminary prospectus)
and such other documents as such seller may reasonably request to facilitate the
disposition of the Registerable Securities;

         (iv) use its best efforts to register or to qualify such Registerable
Securities under such securities or blue sky laws of such jurisdictions as the
Shareholder reasonably requests and do any and all other acts and things which
may be reasonably necessary or advisable to enable such seller to consummate the
disposition in such jurisdictions of the Registerable Securities (provided that
the Company will not be required to (i) qualify generally to do business in any
jurisdiction where it would not otherwise be required to qualify but for this
paragraph, (ii) subject itself to taxation in any such jurisdiction or (iii)
consent to general service of process in any such jurisdiction);

         (v) notify the Shareholder, at any time when a prospectus relating to
the Registerable Securities is required to be delivered under the Securities
Act, of the happening of any event as a result of which the prospectus included
in such registration statement contains an untrue statement of a material fact
or omits any fact necessary to make the statements therein not misleading, and,
at the request of the Shareholder, the Company will prepare a supplement or
amendment to such prospectus so that, as thereafter delivered to the purchasers
of such Registerable Securities, such prospectus will not contain an untrue
statement of a material fact or omit to state any fact necessary to make the
statements therein not misleading;

         (vi) cause all such Registerable Securities to be listed on each
securities exchange on which similar securities issued by the Company are then
listed;

         (vii) provide a transfer agent and registrar for all such Registerable
Securities not later than the effective date of such registration statement; and

                                      -3-
<PAGE>   4

         (viii) in the event of the issuance of any stop order suspending the
effectiveness of a registration statement, or of any order suspending or
preventing the use of any related prospectus or suspending the qualification of
any common stock included in such registration statement for sale in any
jurisdiction, the Company will use its best efforts promptly to obtain the
withdrawal of such order.

         (b) Zonagen shall not have any obligation to enter into an underwriting
agreement with any investment banking firm participating as a broker in the
execution of any sales of Registerable Securities pursuant to a Demand
Registration. In no event shall the plan of distribution with respect to a
Demand Registration include the use of a contractual underwriter.

         (c) The registration rights provided by this Section are for the
benefit solely of the Shareholder, are personal in nature, and shall not be
available to any subsequent holder of the Registrable Securities (other than
subsequent holders who have become such by gift or other transfer by the
Shareholder to an immediate family member of the Shareholder, by will or through
operation of the laws of descent and distribution).

         (d) For as long as the Shareholder shall continue to hold any
Registerable Securities, Zonagen will undertake, on a reasonable commercial
basis, to timely file all annual, quarterly and other reports required to be
filed by it under Section 13 or 15(d) of the Securities Exchange Act of 1934, as
amended (the "Exchange Act"), and the regulations of the Commission thereunder.
In the event of any proposed sale of Shares by the Shareholder pursuant to Rule
144 under the Securities Act, Zonagen shall cooperate with the Shareholder so as
to enable such sales to be made in accordance with applicable laws, rules and
regulations, the requirement of Zonagen's transfer agents, and the reasonable
requirements of the broker through which the sales are proposed to be executed.

         5.       REGISTRATION EXPENSES.

         (a) Except as provided in Section 5(b), all expenses incident to the
Company's performance of or compliance with this Agreement, including without
limitation all registration and filing fees, fees and expenses of compliance
with securities or blue sky laws, panting expenses, messenger and delivery
expenses, and fees and disbursements of counsel for the Company and all
independent certified public accountants, underwriters (excluding discounts and
commissions) and other Persons retained by the Company (all such expenses being
herein called "Registration Expenses"), will be borne as provided in this
Agreement, except that the Company will, in any event, pay its internal expenses
(including, without limitation, all salaries and expenses of its officers and
employees performing legal or accounting duties), the expense of any annual
audit or quarterly review, and the expenses and fees for listing the securities
to be registered on each securities exchange on which similar securities issued
by the Company are then listed. The Shareholder shall pay only the expenses of
its counsel.

                                      -4-
<PAGE>   5

         (b) With respect to a Demand Registration with respect to which no
Short Term Registration is available, the Company will pay all of the expenses
set forth in Section 5(a), except that the Shareholder will pay the expenses of
the Company's legal counsel as incurred.

         6.       INDEMNIFICATION.

         (a) The Company agrees to indemnify the Shareholder, to the extent
permitted by law, against ad losses, claims, damages, liabilities and expenses
caused by any untrue or alleged untrue statement of material fact contained in
any registration statement, prospectus or preliminary prospectus or any
amendment thereof or supplement thereto or any omission or alleged omission of a
material fact required to be stated therein or necessary to make the statements
therein not misleading, except insofar as the same are caused by or contained in
any information furnished in writing to the Company by the Shareholder expressly
for use therein or by the Shareholder's failure to deliver a copy of the
registration statement or prospectus or any amendments or supplements thereto
after the Company has furnished such holder with a sufficient number of copies
of the same. In connection with an underwritten offering, the Company will
indemnify such underwriters, their officers and directors and each person who
controls such underwriters (within the meaning of the Securities Act) to the
same extent as provided above with respect to the indemnification of the
Shareholder.

         (b) In connection with any registration statement in which the
Shareholder is participating, the Shareholder will furnish to the Company in
writing such powers of attorney, custody agreements and letters of direction and
other information and affidavits as the Company reasonably requests for use in
connection with any such registration statement or prospectus and, to the extent
permitted by law, will only indemnify the Company, its directors and officers
and each Person who controls the Company (within the meaning of the Securities
Act) against any losses, claims, damages, liabilities and expenses resulting
from any untrue or alleged untrue statement of material fact contained in the
registration statement, prospectus or preliminary prospectus or any amendment
thereof or supplement thereto or any omission or alleged omission of a material
fact required to be stated therein or necessary to make the statements therein
not misleading, but only to the extent that such untrue statement or omission is
contained in an information or affidavit so furnished in writing by the
Shareholder; provided that the obligation to indemnify will be limited to the
net amount of proceeds received by the Shareholder from the sale of Registerable
Securities pursuant to such registration statement.

         (c) Any Person entitled to indemnification hereunder will (i) give
prompt written notice to the indemnifying party of any claim with respect to
which it seeks indemnification and (ii) unless in such indemnified party's
reasonable judgment a conflict of interest between such indemnified and
indemnifying parties may exist with respect to such claim, permit such
indemnifying party to assume the defense of such claim with counsel reasonably
satisfactory to the indemnified party. If such defense is assumed, the
indemnifying party will not be subject to

                                      -5-
<PAGE>   6

any liability for any settlement made by the indemnified party without its
consent (but such consent will not be unreasonably withheld). An indemnifying
party who is not entitled to, or elects not to, assume the defense of a claim
will not be obligated to pay the fees and expenses of more than one counsel for
all parties indemnified by such indemnifying party with respect to such claim,
unless in the reasonable judgment of any indemnified party a conflict of
interest may exist between such indemnified party and any other of such
indemnified parties with respect to such claim.

         (d) The indemnification provided for under this Agreement will remain
in full force and effect regardless of any investigation made by or on behalf of
the indemnified party or any officer, director or controlling Person of such
indemnified party and will survive the transfer of securities. The Company also
agrees to make such provisions, as are reasonably requested by any indemnified
party, for contribution to such party in the event the Company's indemnification
is unavailable for any reason.

         7. PARTICIPATION IN UNDERWRITTEN REGISTRATIONS. The Shareholder may not
participate in any registration hereunder which is underwritten unless the
Shareholder (i) agrees to sell the Registerable Securities on the basis provided
in any underwriting arrangements approved by the Company and (ii) completes and
executes all questionnaires, powers of attorney, indemnities, underwriting
agreements and other documents required under the terms of such underwriting
arrangements; provided that the Shareholder shall not be required to make any
representations or warranties to the Company or the underwriters other than
representations and warranties regarding such Shareholder and such Shareholder's
intended method of distribution.

         8.       DEFINITIONS.

         "COMMON STOCK" means the Company's common stock, par value $.01 per
share, and (ii) any securities issued or issuable with respect to such Common
Stock by way of stock dividends or stock splits or in connection with a
combination of shares, recapitalization, merger, consolidation, or other
reorganization.

         "PERSON" means an individual, partnership, corporation, joint stock
company, trust, joint venture or unincorporated organization.

         "REGISTERABLE SECURITIES" means any Common Stock issued in satisfaction
of the Final Payment and in satisfaction of the Remaining Shareholder Debt and
any Common Stock issued or issuable with respect to such securities by way of a
stock dividend or stock split or in connection with a combination of shares,
recapitalization, merger, consolidation or other reorganization. As to any
particular Registerable Securities, such securities will cease to be
Registerable Securities when they have been distributed to the public pursuant
to an offering

                                      -6-
<PAGE>   7

registered under the Securities Act or sold to the public through a broker,
dealer or market maker in compliance with Rule 144 under the Securities Act (or
any similar rule then in force).

         9.       MISCELLANEOUS.

         (a) REMEDIES. Any Person having rights under any provision of this
Agreement will be entitled to enforce such rights specifically or to recover
damages caused by reason of any breach of any provision of this Agreement and to
exercise all other rights granted by law. The parties hereto agree and
acknowledge that money damages may not be an adequate remedy for any breach of
the provisions of this Agreement and that any party may in its sole discretion
apply to any court of law or equity of competent jurisdiction (without posting
any bond or other security) for specific performance and for other injunctive
relief in order to enforce or to prevent violation of the provisions of this
Agreement.

         (b) AMENDMENTS AND WAIVERS. Except as otherwise provided herein, the
provisions of this Agreement may be amended or waived only upon the prior
written consent of the parties hereto.

         (c) SUCCESSORS AND ASSIGNS. All covenants and agreements in this
Agreement by or on behalf of any of the parties hereto will bind and inure to
the benefit of the respective successors and assigns of the parties hereto
whether so expressed or not; provided, however, that the Shareholder's rights
hereunder may not be assigned without the consent of the Company, which consent
may be withheld for any reason.

         (d) SEVERABILITY. Whenever possible, each provision of this Agreement
shall be interpreted in such manner as to be effective and valid under
applicable law, but if any provision of this Agreement is held to be prohibited
by or invalid under applicable law, such provision shall be ineffective only to
the extent of such prohibition or invalidity, without invalidating the remainder
of this Agreement.

         (e) COUNTERPARTS. This Agreement may be executed simultaneously in two
or more counterparts, any one of which need not contain the signatures of more
than one party, but all such counterparts taken together shall constitute one
and the same Agreement.

         (f) DESCRIPTIVE HEADINGS. The descriptive headings of this Agreement
are inserted for convenience only and are not a part of this Agreement.

         (g) GOVERNING LAW. This Agreement shall be governed by and interpreted
in accordance with the domestic laws of the State of Texas, without giving
effect to any choice of law or conflict of law provision or rule (whether of the
State of Texas or any other jurisdiction) that would cause the application of
the laws of any jurisdiction other than the State of Texas.

                                      -7-
<PAGE>   8

         (h) NOTICES. All notices, demands or other communications to be given
or delivered under or by reason of the provisions of this Agreement shall be in
writing and shall be deemed to have been given when delivered personally to the
recipient, sent to the recipient by reputable express courier service (charges
prepaid) or mailed to the recipient by certified or registered mail, return
receipt requested and postage prepaid. Such notices, demands and other
communications shall be sent as follows:

                           If to the Company:

                           Zonagen, Inc.
                           2408 Timberloch Place, B-4
                           The Woodlands, Texas 77380
                           Attn: Joseph S. Podolski

                           With a Copy to:

                           Porter & Hedges, L.L.P.
                           700 Louisiana, Suite 3500
                           Houston, Texas 77002
                           Attn: Samuel N. Allen

                           If to the Shareholder:

                           J. Tyler Dean
                           313 Speen Street
                           Natick, Massachusetts 01760

                           With a Copy to:

                           Hargraves, Karb, Wilcox & Galvani
                           The Corporate Center
                           550 Cochituate Road
                           P. O. Box 966
                           Framingham, Massachusetts 01701

                           Attn: William Pezzoni

                                      -8-
<PAGE>   9

                 SIGNATURE PAGE TO REGISTRATION RIGHTS AGREEMENT

The parties hereto have executed this Agreement on the day and year first above
written.

                                       ZONAGEN, INC.

                                       By: /s/ Louis Ploth, Jr.
                                           -------------------------------------
                                       Louis Ploth, Jr., Chief Financial Officer

                                       SHAREHOLDER

                                           /s/ J. Tyler Dean
                                       -----------------------------------------
                                       J. TYLER DEAN

                                      -9-

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