Document:

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                                                                   EXHIBIT 10.22

                          AMENDED SINGLE INSTALLMENT NOTE
    HOUSTON, TEXAS           (PAYMENT OF MEDICARE TAXES)        JULY 1, 2002

This Amendment pertains to that certain Single Installment Note entered into on
or about October 28, 2000 by and between Nathaniel A. Gregory and NATCO Group
Inc. ("NGI") to cover Medicare tax liability resulting from the exercise of
certain stock options. The original note was in the principal amount of
$37,647.36 with interest accruing at an annual rate of 7.8% with payment of the
principal balance and all accrued interest due on or before July 31, 2003.

On July 1, 2002, NGI and Gregory agreed to amend the original note and Gregory
agrees, as follows:

         In consideration of value acknowledged and received, Gregory agrees to
         pay NGI the principal amount of $42,562.97 (i.e., $37,647.36 in initial
         principal plus $4,915.61 of accrued interest) on which interest shall
         accrue at an annual rate based on ninety-day LIBOR plus 300 basis
         points adjusted on the first day of each calendar quarter. All payments
         of principal and interest on this Amended Note shall be paid in the
         legal currency of the United States; and Gregory waives presentment for
         payment, protest, and notice of protest and nonpayment of this Amended
         Note.

         Payment of the principal balance and all accrued interest shall be due
         July 31, 2004. However, Gregory reserves the right to repay this
         Amended Note, in whole or in part, before July 31, 2004 with no
         prepayment penalty.

         Failure to make payment when due shall be considered default. Should
         default exist for more than a grace period of sixty days, Gregory
         agrees to pay NGI all reasonable costs and expenses incurred during
         collection proceedings including, but not limited to, attorney's fees.

         No renewal or extension of this Amended Note, delay in enforcing any
         right of NGI under this Amended Note, or assignment by NGI of this
         Amended Note shall affect the liability or the obligations of Gregory.
         All rights of NGI under this Amended Note are cumulative and may be
         exercised concurrently or consecutively at NGI's option.

         If any one or more of the provisions of this Amended Note are
         determined to be unenforceable, in whole or in part, for any reason,
         the remaining provisions shall remain operative.

         Gregory acknowledges and agrees that this Amended Note is with rights
         of full recourse and that he is responsible for repayment of the
         principal and all accrued interest on or before the maturity date.

         THIS AMENDED NOTE SHALL BE CONSTRUED, GOVERNED, AND INTERPRETED IN
         ACCORDANCE WITH THE APPLICABLE LAWS OF THE STATE OF TEXAS.

                                             /s/ NATHANIEL A. GREGORY
                                            -----------------------------------
                                            Nathaniel A. Gregory

State of Texas       )
County of Harris     )

         SWORN TO AND SUBSCRIBED TO before me on this 30th day of July 2002, by
Nathaniel A. Gregory, a person known to me.

                                          /s/ PATRICIA G. SMITH
                                         --------------------------------------
                                         Notary Public for the State of Texas

         (S E A L)
                    PATRICIA G. SMITH
         [SEAL]   MY COMMISSION EXPIRES
                    AUGUST 14, 2002<PAGE>

                                                                   EXHIBIT 10.23

                         AMENDED SINGLE INSTALLMENT NOTE
    HOUSTON, TEXAS          (PAYMENT OF INCOME TAXES)          JULY 1, 2002

This Amendment pertains to that certain Single Installment Note entered into on
or about April 15, 2002 by and between Patrick M. McCarthy and NATCO Group Inc.
("NGI") to cover income tax liability resulting from the exercise of certain
stock options. The original note was in the principal amount of $216,297.00 with
interest accruing at an annual rate of 6.0% with payment of the principal
balance and all accrued interest due on or before July 31, 2003.

On July 1, 2002, NGI and McCarthy recently agreed to amend the original note and
McCarthy agrees, as follows:

         In consideration of value acknowledged and received, McCarthy agrees to
         pay NGI the principal amount of $218,999.23 (i.e., $216,297.00 in
         initial principal plus $2,702.23 of accrued interest) on which interest
         shall accrue at an annual rate based on ninety-day LIBOR plus 300 basis
         points adjusted on the first day of each calendar quarter. All payments
         of principal and interest on this Amended Note shall be paid in the
         legal currency of the United States; and McCarthy waives presentment
         for payment, protest, and notice of protest and nonpayment of this
         Amended Note.

         Payment of the principal balance and all accrued interest shall be due
         July 31, 2004. However, McCarthy reserves the right to repay this
         Amended Note, in whole or in part, before July 31, 2004 with no
         prepayment penalty.

         Failure to make payment when due shall be considered default. Should
         default exist for more than a grace period of sixty days, McCarthy
         agrees to pay NGI all reasonable costs and expenses incurred during
         collection proceedings including, but not limited to, attorney's fees.

         No renewal or extension of this Amended Note, delay in enforcing any
         right of NGI under this Amended Note, or assignment by NGI of this
         Amended Note shall affect the liability or the obligations of
         McCarthy. All rights of NGI under this Amended Note are cumulative and
         may be exercised concurrently or consecutively at NGI's option.

         If any one or more of the provisions of this Amended Note are
         determined to be unenforceable, in whole or in part, for any reason,
         the remaining provisions shall remain operative.

         McCarthy acknowledges and agrees that this Amended Note is with rights
         of full recourse and that he is responsible for repayment of the
         principal and all accrued interest on or before the maturity date.

         THIS AMENDED NOTE SHALL BE CONSTRUED, GOVERNED, AND INTERPRETED IN
         ACCORDANCE WITH THE APPLICABLE LAWS OF THE STATE OF TEXAS.

                                             /s/ PATRICK M. McCARTHY
                                            -----------------------------------
                                            Patrick M. McCarthy

State of Texas     )
County of Harris   )

         SWORN TO AND SUBSCRIBED TO before me on this 18th day of July 2002, by
Patrick M. McCarthy, a person known to me.

                                          /s/ PATRICIA G. SMITH
                                         --------------------------------------
                                         Notary Public for the State of Texas

         (S E A L)
                    PATRICIA G. SMITH
         [SEAL]   MY COMMISSION EXPIRES
                    AUGUST 14, 2002<PAGE>
                                                                    EXHIBIT 10.1

Portions of this Exhibit have been omitted pursuant to a request for
confidential treatment under Rule 24b-2 of the Securities Exchange Act of 1934,
as amended (the "Exchange Act"). The omitted portions are marked [ ** ] and have
been filed separately with the Securities and Exchange Commission (the
"Commission").

                        [SCHERING CORPORATION LETTERHEAD]

                                                       July 10, 2002

Mr. Joseph Podolski
President & CEO
Zonagen, Inc.
2408 Timberloch Place, B-4
The Woodlands, Texas 77380

Dear Mr. Podolski:

With respect to the Exclusive License Agreement, dated as of November 14, 1997,
by and between Zonagen, Inc. ("Zonagen") and Schering Corporation ("Schering")
relating to the U.S. license of Vasomax(TM) (the "US Agreement") and the
Exclusive License Agreement, dated as of November 14, 1997, by and between
Zonagen and Schering-Plough Ltd. (together with Schering, "SP") relating to the
non-U.S. license of Vasomax(TM) (together with the US Agreement, the
"Agreements"), it is understood and agreed by the parties that it is no longer
in their respective best interest to continue under the Agreements. This Letter
of Agreement sets forth the terms and conditions on which the parties have
mutually agreed to terminate the Agreements.

The parties agree that effective as of the date upon which this Letter of
Agreement is countersigned by Zonagen (the "Termination Date"), the Agreements
are hereby terminated by mutual written agreement of the parties in accordance
with section 13.1 of each of the Agreements. Notwithstanding anything in either
of the Agreements to the contrary, the rights and obligations of the parties
following such termination shall be governed solely by the terms and conditions
set forth in this Letter of Agreement.

1)        DEFINITIONS: The following capitalized terms as used in this Letter of
          Agreement, whether used in the singular or plural, shall have the
          meanings set forth below. Capitalized terms used herein which are not
          expressly defined in this Letter of Agreement shall have the meanings
          set forth in the Agreements.

          1.1  "Revenue" shall mean with respect to Vasomax(TM): (i) Zonagen Net
               Sales of the product, less Cost-of-Goods Sold and Zonagen's
               reasonable out-of-pocket costs for patents and trademarks; and/or
               (ii) the aggregate total of license fees, up-front payments,
               milestone payments, royalties or other forms of payment received
               by Zonagen (or its successor in interest) in connection with any
               license, distributorship, assignment, transfer or other grant or
               conveyance of rights in or to the product.

<PAGE>
          1.2  "Net Royalty Revenue" shall mean with respect to Combination
               Products: (i) Zonagen Net Sales of the Combination Product, less
               Cost-of-Goods Sold and Zonagen's reasonable out-of-pocket costs
               for patents and trademarks; and/or (ii) royalties received by
               Zonagen (or its successor in interest) in connection with any
               license, distributorship, assignment, transfer or other grant or
               conveyance of rights in or to the Combination Product.

          1.3  "Zonagen Net Sales" shall mean the proceeds actually received by
               Zonagen (or its successor in interest) and/or its Affiliates on
               all sales of a given product to an unaffiliated third party
               (whether an end-user, a distributor or otherwise), and exclusive
               of intercompany transfers or sales among Zonagen and its
               affiliates (if any), less the reasonable and customary deductions
               from such gross amounts for: (i) credits or allowances actually
               granted for damaged goods, returns or rejections of product and
               retroactive price reductions; (ii) sales or similar taxes
               (including duties or other governmental charges levied on,
               absorbed or otherwise imposed on the sale of product); (iii)
               freight, postage, shipping, customs duties and insurance charges
               to the extent included in the proceeds actually received from the
               customer; and (iv) charge-back payments and rebates granted to
               managed health care organizations or to federal, state and local
               governments, their agencies, purchasers and reimbursers, or to
               trade customers, including without limitation wholesalers and
               chain and pharmacy buying groups.

          1.4  "Cost-of-Goods Sold" shall mean (i) Zonagen's fully burdened cost
               of manufacturing the product as determined in accordance with
               generally accepted accounting principles, or (ii) if the product
               is manufactured for Zonagen by a third party contract
               manufacturer, the actual price paid by Zonagen for the purchase
               of the product from such contract manufacturer.

          1.5  "Licensed Patent Rights" shall mean all Patent Rights (as defined
               in the Agreements) existing as of the Termination Date and having
               one or more claims related to Vasomax(TM), as listed on Schedule
               A (attached hereto), that are assigned to SP and/or with respect
               to which SP is responsible for the filing, prosecution,
               maintenance, enforcement or defense.

          1.6  "Zonagen Patent Rights" shall mean all Patent Rights (as defined
               in the Agreements) existing as of the Termination Date and which
               are not Licensed Patent Rights.

          1.7  "Combination Patents" shall mean those granted patents existing
               as of the Termination Date which are owned or controlled by SP
               and have one or more claims covering Combination Products and/or
               the use thereof, as listed in Schedule B (attached hereto).

          1.8  "Combination Product" shall mean any pharmaceutical composition
               or product containing a combination of phentolamine and
               apomorphine as active ingredients.

<PAGE>
[  **  ] This portion has been omitted based on a request for confidential
treatment pursuant to Rule 24b-2 of the Exchange Act. The omitted portion has
been separately filed with the Commission.

          1.9  "Valid Claim" shall mean a claim of an issued and unexpired
               patent which has not been revoked or held unenforceable or
               invalid by a decision of a court or other governmental agency of
               competent jurisdiction, unappealable or unappealed within the
               time allowed for appeal, and which has not been disclaimed,
               denied or admitted to be invalid or unenforceable through reissue
               or disclaimer or otherwise.

2)        TREATMENT OF LICENSES, OPTIONS AND EXISTING DEBT UNDER THE
          AGREEMENTS.

          2.1  TERMINATION OF LICENSES. All licenses, options and other rights
               with respect to Licensed Compounds, Licensed Products, Zonagen
               Combination Products, Patent Rights, Zonagen Know-How and Zonagen
               Trademarks granted to SP by Zonagen under sections 2.1, 2.2, 3.1
               and 3.2 of each of the Agreements are hereby terminated and shall
               revert to Zonagen in their entirety.

          2.2  FEMALE INDICATION. SP shall retain no rights with respect to the
               Female Indication or the female applications of Vasomax(TM), and
               in the event such indication is successfully developed by
               Zonagen, following regulatory approval of Vasomax(TM) for that
               indication, Zonagen shall have no financial obligations to SP
               under this Letter of Agreement, including under paragraphs, 5, 6
               or 8 with respect to any sales of Vasomax(TM) that are directly
               attributable to the Female Indication, as demonstrated by
               Zonagen's written business records.

          2.3  TREATMENT OF EXISTING DEBT. The parties hereby acknowledge and
               agree that any payments owed by one party to the other party
               under the Agreements prior to the Termination Date (including but
               not limited to the [ ** ] owed by Zonagen to SP relating to
               expenses incurred under certain clinical trials) are hereby
               forever discharged, released and satisfied by the other party's
               agreement to the terms of this Letter of Agreement.

3)        OTHER TRANSFERS.

          3.1  TRANSFER OF RESPONSIBILITY. Effective as of the Termination Date,
               Zonagen shall be solely responsible, at its expense, for all
               aspects of the development and commercialization of the Licensed
               Product currently being developed and commercialized under the
               trademark Vasomax(TM) (hereinafter referred to as "Vasomax(TM)"),
               and SP and its Affiliates shall have no further obligations to
               Zonagen under the Agreements with respect thereto. SP shall
               reasonably cooperate with Zonagen to facilitate the transfer of
               such responsibilities to Zonagen in a timely manner, including as
               set forth in paragraph 4, below. Notwithstanding the above,
               nothing herein is intended to, nor shall it, obligate Zonagen to
               expend any funds or conduct any activities relating to
               Vasomax(TM), all such actions being at the sole discretion of
               Zonagen.

          3.2  HEALTH REGISTRATIONS. On or promptly after the Termination Date,
               SP will transfer and assign to Zonagen (or its designee) all
               rights, title and interest in and to any and all NDAs and/or HRDs
               for Vasomax(TM). Zonagen (or its designee) will

<PAGE>
[  **  ] This portion has been omitted based on a request for confidential
treatment pursuant to Rule 24b-2 of the Exchange Act. The omitted portion has
been separately filed with the Commission.

               thereafter be solely responsible for any and all obligations in
               the relevant countries with respect to such NDAs and/or HRDs,
               including without limitation, all reporting obligations and
               compliance with all applicable laws, rules and regulations with
               respect thereto. Notwithstanding the above, nothing herein is
               intended to, nor shall it, obligate Zonagen to expend any funds
               or conduct any activities relating to Vasomax(TM), all such
               actions being at the sole discretion of Zonagen.

          3.3  LICENSED PATENTS. On or promptly after the Termination Date, SP
               will transfer and assign to Zonagen or its designee all of SP's
               right, title and interest in Licensed Patent Rights. Zonagen will
               thereafter be solely responsible, at Zonagen's expense, for the
               prosecution, maintenance, enforcement and defense of the Licensed
               Patent Rights. Notwithstanding the above, nothing herein is
               intended to, nor shall it, obligate Zonagen to expend any funds
               or conduct any activities relating to Vasomax(TM), all such
               actions being at the sole discretion of Zonagen.

4)        INITIAL PAYMENT. Zonagen shall pay [ ** ] to SP within ten (10)
          business days after the Termination Date. Such payment shall be made
          by wire transfer in immediately available funds to such bank account
          as is designated in writing by SP.

5)        MILESTONE PAYMENT. In the event that worldwide annual sales of
          Vasomax(TM) during any calendar year exceeds [ ** ], Zonagen shall pay
          to SP a one-time milestone payment of [ ** ]. Such milestone payment
          shall be payable within ninety (90) days after the end of the relevant
          calendar year in which such sales occur; provided, however, that if
          Zonagen's worldwide Revenue for Vasomax(TM) in that calendar year are
          less than [ ** ], Zonagen shall pay to SP an amount equal to [ ** ] of
          Zonagen's worldwide Revenue for Vasomax(TM) for that calendar year,
          and within ninety (90) days after the end of each subsequent calendar
          year thereafter shall pay to SP an amount equal to the lesser of (i)
          the unpaid portion of such milestone payment, or (ii) [ ** ] of
          Zonagen's Revenue for Vasomax in the relevant calendar year, until
          such time as the aggregate total of all such payments to SP is equal
          to [ ** ]. Upon payment of an aggregate of [ ** ] under this paragraph
          5, Zonagen shall have no further obligations to make any payments
          under this paragraph 5.

6)       VASOMAX(TM) ROYALTIES. Subject to the limitations set forth below in
         paragraph 6.1, Zonagen shall pay to SP royalties on Zonagen Net Sales
         of Vasomax(TM) as follows:

          6.1  In those countries where as of the Termination Date there are
               existing one or more granted patents within the Zonagen Patent
               Rights or Licensed Patent Rights, a royalty of [ ** ] of Zonagen
               Net Sales of Vasomax(TM) in such countries on cumulative annual
               Zonagen Net Sales in those countries of up to [ ** ], and [ ** ]
               on that portion of cumulative annual Zonagen Net Sales of
               Vasomax(TM) in those countries in excess of [ ** ]. Such royalty
               obligations will expire in such countries upon the earlier of (i)
               expiration of the last to expire of the granted Zonagen Patent
               Rights and Licensed Patent Rights existing in such countries on

<PAGE>
[  **  ] This portion has been omitted based on a request for confidential
treatment pursuant to Rule 24b-2 of the Exchange Act. The omitted portion has
been separately filed with the Commission.

               the Termination Date, or (ii) on the ten (10) year anniversary of
               the Termination Date.

          6.2  A royalty of [ ** ] of annual Zonagen Net Sales of Vasomax(TM) in
               Brazil; provided that no royalties shall be payable on annual
               Zonagen Net Sales in Brazil in excess of the Prior Sales
               Threshold (as defined below). The term "Prior Sales Threshold"
               shall mean the annual Zonagen Net Sales of Vasomax(TM) in Brazil
               during the twelve (12) month period immediately prior to the
               Termination Date. No royalties shall be payable under paragraph
               6.1 with respect to sales of Vasomax(TM) in Brazil.

          6.3  No royalties shall be payable in countries where, as of the
               Termination Date, there are no granted patents within the Zonagen
               Patent Rights or Licensed Patent Rights in such country,
               irrespective or whether or not any such patents are granted at
               any time after the Termination Date.

          6.4  Zonagen's annual royalty obligations to SP under paragraphs 6.1
               and 6.2 above for sales of Vasomax(TM) in a given country shall
               be capped at an amount not to exceed [ ** ] of the cumulative
               Revenue received by Zonagen, or its successor in interest, based
               upon cumulative annual sales of Vasomax(TM) in such countries
               during the same year.

          6.5  Zonagen's cumulative aggregate royalty obligation to SP with
               respect to Vasomax(TM) shall be capped at [ ** ]. The initial
               payment and milestone payments provided for in paragraphs 4 and
               5, and any payments received by SP in respect of Combination
               Patents pursuant to paragraph 8, shall not be included in
               determining whether not the cap has been reached.

7)        LICENSE TO COMBINATION PATENTS. SP hereby grants to Zonagen (or its
          designee) an exclusive, worldwide, royalty-bearing license under all
          of SP's right, title and interest in the Combination Patents. SP will
          retain responsibility for the prosecution, maintenance, defense and
          enforcement of the Combination Patents, at its expense and in its sole
          discretion. In the event that SP intends to abandon one or more of the
          Combination Patents, it will first offer to assign and transfer such
          patents to Zonagen.

8)        COMBINATION PRODUCT ROYALTIES. In consideration for the license to
          Combination Patents granted by SP to Zonagen under paragraph 7,
          Zonagen shall pay royalties to SP as follows:

          8.1  Zonagen shall pay to SP an amount equal to [ ** ] of Zonagen's
               annual Net Royalty Revenues with respect to any sales of
               Combination Product(s) in those countries where and for so long
               as the making, using and/or selling of such Combination
               Product(s) is covered by one or more Valid Claims of a
               Combination Patent. Such amounts shall be payable within ninety
               (90) days after the end of each calendar year; provided that
               Zonagen shall have no payment obligations to SP under this
               paragraph 8.1 for sales of Combination Product(s) in

<PAGE>
[  **  ] This portion has been omitted based on a request for confidential
treatment pursuant to Rule 24b-2 of the Exchange Act. The omitted portion has
been separately filed with the Commission.

               any country(ies) during any calendar year (or portion thereof)
               where the Combination Product is not covered by a Combination
               Patent.

          8.2  In the event that Zonagen licenses, assigns, transfers or
               otherwise conveys the rights in and to the Combination Patents to
               any third party in exchange for one or more lump sum payments (in
               lieu of running royalties), Zonagen shall pay to SP an amount
               equal to [ ** ] of all such payments. Each such payment shall be
               payable within thirty (30) days after receipt of the relevant
               lump sum payment by Zonagen from such third party.

          8.3  In the event that Zonagen is acquired (as defined below) by an
               independent third party, the acquiring party shall pay to SP a
               royalty equal to [ ** ] of annual net sales by or on behalf of
               such acquiring party or its affiliates of any Combination
               Products in those countries where the Combination Product is
               covered by at least one claim of a Combination Patent. Such
               royalty payments shall be payable within ninety (90) days after
               the end of each calendar year. Net sales of such Combination
               Product by or on behalf of such acquiring party or its affiliates
               shall be determined in an analogous manner to Zonagen Net Sales.
               An independent third party shall "acquire" Zonagen if such third
               party (i) directly or indirectly acquires ownership of securities
               representing more then fifty percent (50%) of Zonagen's
               then-outstanding shares of capital stock, or the combined voting
               power of Zonagen's then-outstanding voting securities, (ii) is
               the surviving entity in any merger, reorganization, consolidation
               or combination involving Zonagen, or (iii) purchases or otherwise
               acquires all or substantially all of Zonagen's assets.

          8.4  Zonagen's payment obligations to SP with respect to Combination
               Patents under paragraphs 8.1, 8.2 and 8.3 shall not be subject to
               the royalty cap for Vasomax(TM) products described above in
               paragraph 6.5, and any payments made by Zonagen (or its successor
               in interest) to SP in consideration for the licenses to
               Combination Patents granted in paragraph 7 shall not be
               considered in calculation of such royalty cap.

9)        RECORD KEEPING. Zonagen and its Affiliates shall keep complete and
          accurate records in sufficient detail to enable the royalties and
          other amounts payable to SP hereunder to be determined. Upon
          forty-five (45) days prior written notice from SP, Zonagen shall
          permit an independent certified public accounting firm of nationally
          recognized standing selected by SP and reasonably acceptable to
          Zonagen, at SP's expense, to have access during normal business hours
          to examine pertinent books and records of Zonagen and/or its
          Affiliates as may be reasonably necessary to verify the accuracy of
          Zonagen's records with respect to any amounts payable to SP under this
          Letter of Agreement. Any such examination shall not occur more than
          once in any calendar year and shall be limited to pertinent books and
          records for any calendar year ending not more than twenty-four (24)
          months prior to the date of such request. The independent accounting
          firm shall enter into a confidentiality agreement (in form and
          substance reasonably acceptable to Zonagen) as to any of Zonagen's or
          its Affiliates' confidential information to which they are provided
          access while conducting any audit pursuant to this paragraph 9.
          Zonagen may designate competitively sensitive information which the
          auditor may not disclose to

<PAGE>

          SP; provided, however, that such designation shall not encompass the
          auditor's conclusions. The auditor shall disclose to SP only whether
          the amounts paid to SP by Zonagen under this Letter of Agreement are
          correct or incorrect, and the specific details concerning any
          discrepancies. No other information shall be provided to SP by the
          auditors.

10)       RELEASES AND SETTLEMENTS OF ALL CLAIMS.

          10.1 The parties acknowledge and agree on behalf of themselves and
               their respective Affiliates that this Letter of Agreement shall
               constitute a final and binding settlement between the parties of
               any and all claims and/or disputes between the parties existing
               as of the Termination Date (including without limitation any and
               all potential claims and disputes which either party could have
               made as of the Termination Date) with regard to each party's
               performance of, or failure to perform, activities related to or
               in connection with the development and commercialization of
               Vasomax(TM), or otherwise under the Agreements.

          10.2 Zonagen, on behalf of itself and its Affiliates, hereby releases
               and discharges SP, its Affiliates, and its and their respective
               officers, directors, employees and agents from all causes of
               action, demands, claims, damages and liabilities arising from
               acts occurring prior to the Termination Date with respect to (i)
               the development and commercialization of Vasomax(TM) by SP and
               its Affiliates, (ii) the filing, prosecution, maintenance,
               defense and enforcement by SP or its Affiliates of the Licensed
               Patent Rights and/or Combination Patents.

          10.3 SP, on behalf of itself and its Affiliates, hereby releases and
               discharges Zonagen, its Affiliates, and its and their respective
               officers, directors, employees and agents from all causes of
               action, demands, claims, damages and liabilities arising from
               acts occurring prior to the Termination Date with respect to (i)
               the development and commercialization of Vasomax(TM) by Zonagen
               and its Affiliates, (ii) the filing, prosecution, maintenance,
               defense and enforcement by Zonagen or its Affiliates of the
               Zonagen Patent Rights and/or Zonagen Trademarks.

11)       MISCELLANEOUS.

          11.1 CHANGES TO SURVIVING TERMS. The terms of section 13.4 of each of
               the Agreements shall not survive the termination of Agreements
               pursuant to this Letter of Agreement, and the parties agree that
               such provisions are hereby considered null and void.

          11.2 CONFIDENTIALITY. The terms of sections 9.1, 9.2 and 9.3 of each
               of the Agreements shall survive the termination of the Agreements
               and remain in full force and effect.

          11.3 INDEMNIFICATION AND LIABILITY. The terms of Article XI of each of
               the Agreements shall survive the termination of the Agreements
               and remain in effect, subject to the terms and conditions of
               paragraph 10 of this Letter of Agreement. In the event of any
               conflict between the terms of Article XI of each of the

<PAGE>

               Agreements and terms of this Letter of Agreement, the terms of
               the Letter of Agreement shall control.

12)       The terms and existence of this Letter of Agreement shall be governed
          by the nondisclosure terms set froth in section 9.3 of each of the
          Agreements.

This Letter of Agreement together with any Schedules referred to herein and
attached hereto, constitutes the entire agreement between the parties with
respect to the termination of the Agreements and the right and obligations of
the parties and their Affiliates in connection therewith.

No provision of this Letter of Agreement may be modified or amended except
expressly in a writing signed by all of the parties, nor shall any terms be
waived except expressly in a writing signed by the party charged therewith. This
Letter of agreement shall be governed in accordance with the laws of the State
of New Jersey, without regard to principles of conflicts of laws.

Please indicate Zonagen's understanding and agreement to the terms set forth in
this Letter of Agreement by counter-signing the duplicate originals of this
letter and returning one (1) original.

Very truly yours,

SCHERING CORPORATION                         SCHERING-PLOUGH LTD.

By:   /s/ David Poorvin                      By: /s/ David Poorvin
   ----------------------------                 --------------------------------
       David Poorvin, Ph.D.                         David Poorvin, Ph.D.
       Vice President                               Prokurist

Date:  10 July 2002                          Date:  10 July 2002
   ----------------------------                   ------------------------------

AGREED AND UNDERSTOOD:
ZONAGEN, INC.

By:   /s/ Joseph Podolski
   ----------------------------
         Joseph Podolski
         President & CEO

Date:  July 11, 2002
   ----------------------------

<PAGE>
                                   SCHEDULE A

                         LIST OF LICENSED PATENT RIGHTS

<PAGE>
                                   SCHEDULE B

                           LIST OF COMBINATION PATENTS

                   US Patent No. 6,136,818 (disclaimer filed)

                   US Patent No. 6,011,043 (disclaimer filed)

                   US Patent No. 6,001,845

                   And any granted foreign patents existing as of the
                   Termination  Date  corresponding to any of the foregoing US
                   Patents

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00042-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00042-of-00352.parquet"}], [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00042-of-00352.parquet"}]]