Document:

EXHIBIT 10.16

                      AGREEMENT AMONG SHAREHOLDERS
                                   OF
                             ENTROPIN, INC.

Board of Directors
Entropin, Inc.
45926 Oasis Street
Indio, California 92201

Dear Sirs:

This letter is being delivered to you in connection with further financing
obligations undertaken subsequent to the filing of the Form SB-2
Registration Statement (the "Registration Statement") filed by Entropin,
Inc., (the "Company") with the United States Securities and Exchange
Commission (the "SEC").  Since the Registration Statement was declared
effective by the SEC on August 18, 1998 (the "Effective Date"), the
Company's Common Stock shares registered thereby (including those of the
undersigned) have been free-trading without restriction or limitation.

The Company has advised the undersigned that securities underwriters and
other sources of potential future funding of the Company may require that
the undersigned and other shareholders set forth below agree to extend the
term of the previous Agreement Among Shareholders dated June 29, 1998.
The Company has requested the undersigned to agree thereto.

Therefore, in consideration of each of the shareholders set forth below
entering into this Agreement, the undersigned agrees not to offer, sell or
contract to sell or otherwise dispose of, directly or indirectly, or
announce an offering of, any shares of the Company's Common Stock owned by
the undersigned (or any securities convertible into, or exchangeable for,
shares of the Company's Common Stock) during the period ending March 15, 2000.

In the event that, during the term of this Agreement, the Company permits
the sale or other disposition of a portion of shares (a "Permitted
Disposition") by the undersigned shareholders, it shall do so on a PRO RATA
basis.  Since the shares are being registered in the Registration
Statement, the shares are free-trading upon a Permitted Disposition
during the term of this Agreement, or a disposition after the expiration of
this Agreement, provided that the Registration Statement is current at the
time of a disposition.  The Company hereby undertakes to use its best
efforts to keep the Registration Statement current.

Yours very truly,

Date: 3-3-99                  /s/ HIGGINS D. BAILEY /s/ SHIRLEY A. BAILEY
     ----------------         -------------------------------------------
                              Higgins D. Bailey and Shirley A. Bailey

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Date:
     ----------------         -------------------------------------------
                              Higgins D. Bailey, Pledgee

Date:                         /s/ JAMES E. WYNN /s/ JOYCE S. WYNN
     ----------------         -------------------------------------------
                              James E. Wynn and Joyce S. Wynn
                              Accepts this Agreement, providing that
                              during its term he retains the right to sell
                              50,000 shares.

Date:                         /s/ DONALD HUNTER
     ----------------         -------------------------------------------
                              Donald Hunter,
                              Trustee for the Donald Hunter Residuary
                              Marital Trust

Date:
     ----------------         -------------------------------------------
                              Dewey H. and Virginia Crim

Date: 4 Mar 1999              /s/ CAROLINE T. SOMERS
     ----------------         -------------------------------------------
                              Caroline T. Somers
                              I agree to the lock-up agreement providing
                              that the sale of 25,000 shares of my stock
                              at $4.00 per share, for $100,000 total, is
                              completed.

                              CapMac Eighty-Two Limited Partnership,

Date:                         By: /s/ MILTON D. MCKENZIE
     ----------------         -------------------------------------------
                              Milton D. McKenzie, General Partner

Date:                         /s/ MILTON D. MCKENZIE
     ----------------         -------------------------------------------
                              Milton D. McKenzie, Pledgee

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<PAGE>

                              Thomas T. Anderson Trust,

Date: 3/16/99                 By: /s/ THOMAS T. ANDERSON
     ----------------         -------------------------------------------
                              Thomas T. Anderson, Trustee

     AGREED TO AND ACCEPTED BY ENTROPIN, INC.

Date: 3-3-99                  By: /s/ HIGGINS D. BAILEY
     ----------------         -------------------------------------------
                              Higgins D. Bailey,
                              Chairman of the Board

                                    3EXHIBIT 10.17

                                AMENDMENT

     This is  an Amendment ("Amendment") dated August 3, 1998, to that
certain Agreement dated April 6, 1998 by and between Entropin, Inc.
("Entropin"),  a Colorado corporation and Western Center for Clinical
Studies, Inc. ("WCCS"), a California corporation.

     Article IV of the Agreement entitled "COMPENSATION TO WCCS" is hereby
amended to include the following provision:

     4.3  ACCELERATION OF COMPENSATION: In the event that WCCS performs all
          of the terms and conditions of this Agreement for which it is
          obligated prior to January 5, 2001 (the "Completion Date"):  (i)
          all payments pursuant to Section 4.1 shall be accelerated to the
          Completion Date; and, (ii) the Exercise Date of any stock options
          set forth in Section 4.2 which is subsequent to the Completion
          Date shall be accelerated to the Completion Date.

     Article VI of the Agreement entitled "DIRECTORS AND OFFICERS LIABILITY
INSURANCE" is hereby amended as follows:

     6.1  DIRECTORS AND OFFICERS LIABILITY INSURANCE:  During the Term,
          Entropin shall maintain directors and officers insurance in the
          aggregate amount of not less than Three Million Dollars
          ($3,000,000.00).  Such directors and officers insurance policy
          shall be a claims made policy.  If at any time the directors and
          officers insurance policy required hereby is not in effect, none
          of WCCS personnel, Daniel L.  Azarnoff, M.D., Roy S. Azarnoff,
          Ph.D., or Lois Rezler, Ph.D., shall have any obligation or duty
          to

<PAGE>

          perform any services for Entropin in their capacity as an officer
          or director as contemplated by paragraph 3.2 of this Agreement.
          Entropin agrees to maintain "tail" insurance covering Drs.
          Azarnoff and Rezler for not less than two (2) years after the
          termination of this Agreement.

     6.2  INDEMNIFICATION: WCCS agrees to indemnify, defend, and hold
          harmless Entropin, its employees, subcontractors or agents, for
          any and all claims, demands and expenses, including litigation
          costs, arising out of, or in any way resulting from or connected
          with the services or work provided by WCCS, its employees,
          subcontractors or agents.

          It is agreed that this indemnity agreement shall include any
          claim, demand or expense incurred arising out of, resulting from
          or in any way connected with the services provided by WCCS, its
          employees, subcontractors or agents for which WCCS, its
          employees, subcontractors, or agents is solely legally liable.

          It is agreed that WCCS shall carry a general liability policy and
          a products liability policy, each naming Entropin, its employees,
          subcontractors and agents as additional Insureds.  The policies
          shall be in force for a period of not less than two (2) years
          after the termination of the Agreement and shall have limits of
          not less than $2 million dollars.  It is the responsibility of
          WCCS to provide Entropin with the Certificate of Insurance and
          certified copies of the policies.  In the event that the
          aggregate limit of either policy is reduced below the agreed
          limit of $2 million dollars, it is the responsibility of WCCS to
          purchase additional limits to comply with the terms of this
          Agreement.

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<PAGE>

IN WITNESS WHEREOF, the undersigned have entered into this AMENDMENT
effective August 3, 1998.

WESTERN CENTER FOR CLINICAL
   STUDIES, INC.                        ENTROPIN, INC.

By:        /s/ LOIS REZLER              By:  /s/ HIGGINS D. BAILEY
          --------------------------         --------------------------
          Lois Rezler                        Higgins D. Bailey
          President                          Chairman

Date: 8/3/98                            Date: 8-3-98

                                    3EXHIBIT 10.18

                            SECOND AMENDMENT

     This is a Second Amendment ("Amendment") dated July 21, 1999, to that
certain Agreement dated April 6, 1998, by and between Entropin, Inc.
("Entropin"),  a Colorado corporation and Western Center for Clinical
Studies, Inc. ("WCCS"), a California corporation.

     1.    Article II of the Agreement entitled "TERM OF AGREEMENT" is
hereby amended as follows:

          This Agreement shall be for a term commencing as of April 6, 1998
     and through the completion of the NDA phase: provided, however, (1)
     the parties negotiating in good faith, can agree on terms and payment
     commensurate with the agreed upon scope of work for WCCS to supply
     support for the NDA phase; and, (2) either party shall have the right
     to terminate this Agreement prior to such date as set forth in Article
     VII of this Agreement.  This time period shall be known as the "Term".

     2.   Article III of the Agreement entitled "OBLIGATIONS OF WCCS AND
ENTROPIN" is hereby amended as follows:

          3.4   TIME COMMITMENTS: Unless otherwise agreed to in writing by
          Entropin, Lois Rezler, Ph.D., Roy S. Azarnoff, Ph.D., and Daniel
          L. Azarnoff, M.D. shall, during the Term, devote the effort
          necessary and prudent to assure that their obligations under this
          Agreement are completed within the Time Line and Budget. [THE
          REMAINING SENTENCES CONTAINED IN PARAGRAPH 3.4 HAVE BEEN
          ELIMINATED]

          3.7  REPORTS: WCCS shall keep Entropin fully informed of all
          activities undertaken by it for the benefit of Entropin and its
          progress in completing its obligations under this Agreement.  In
          that regard, WCCS shall provide Entropin with progress reports

<PAGE>

          as follows: (i) in compliance with the reporting requirements to
          be determined by the parties no later than August 15, 1999; and,
          (ii) a variance report when its progress relative to the Scope of
          Work, Time Line and Budget varies to a material degree. Entropin
          shall keep WCCS fully informed regarding its efforts to raise
          additional funds.  WCCS shall not disclose to any third party any
          non-public information relating to the financial condition of
          Entropin or its efforts to raise additional funds and neither it
          nor any of its assigns shall at any time buy or sell any shares
          of stock of Entropin based on any such non-public financial
          information.

          3.8  ENTROPIN OBLIGATIONS:

          b.  Entropin will establish a formal review process satisfactory
          to both parties, to ensure that all public relations materials,
          news releases, presentations or other information disseminated to
          the public which refer to the officers, employees or Scope of
          Work of WCCS, or to matters relating to the FDA, DEA, OSHA, and
          other related regulatory agencies, will be presented to WCCS for
          review and approval prior to release.

          3.9  CRO ACTIVITIES: The parties will negotiate in good faith a
          separate agreement regarding the retention of WCCS to perform the
          wrap around scope of work of CRO activities for Phase III A and
          III B.

     3.   Article IV of the Agreement entitled "COMPENSATION TO WCCS" is
hereby amended as follows:

          4.1   PAYMENTS: Subject to any subsequent modification of the
          term and scope of this Agreement, Entropin shall make payment for
          management fees to WCCS in

                                    2

<PAGE>

          the aggregate amount of Eight Hundred Eighty Thousand Four
          Hundred Dollars ($880,400.00), for the period from April 6, 1998,
          through January 5, 2001, and Seventy-six Thousand Four Hundred
          Dollars ($76,400.00) per quarter commencing January, 2001 and
          continuing until NDA submission.  The first such payment shall be
          made concurrently with the execution of this Agreement by
          authorized representatives of WCCS and Entropin and shall be in
          the amount of Forty Thousand Dollars ($40,000).  Thereafter,
          payments shall be made to WCCS as set forth in Exhibit A.
          Promptly upon execution of this Amendment, Entropin shall accrue
          and escrow an amount equivalent to three (3) months of management
          fees and Entropin agrees that such funds identified and dedicated
          to WCCS for services under this Agreement may not nor will be
          used for any purpose other than payment of such services without
          prior written consent from WCCS.  In the event the parties enter
          an agreement concerning the NDA as set forth in Article II of the
          Agreement, the management fees will be increased accordingly

          4.2  STOCK OPTIONS:

          b.   OPTION TERM: The term of each option shall be for seven (7)
          years from the date of its grant.

          d.   EXERCISE DATES:  Subject to Article V of this Agreement,
          WCCS or its assignees may exercise the options granted to it
          under this Agreement according to the schedule listed below:
          provided, however, in the case of any merger or consolidation of
          Entropin with or into another corporation or any other type of
          reorganization which results in a change of control whereby
          Entropin's board of directors prior to

                                    3

<PAGE>

          the reorganization represents less than a majority of Entropin's
          board of directors after the reorganization, all remaining
          options shall become immediately exercisable at the effective
          date of such merger, consolidation or change of control:

               Number of Options             Date of Vesting
               -----------------             ---------------

                     75,000             Fully vested

                    100,000             Start of Phase III Clinical Trials

                     75,000             Completion of Phase III, Part A,
                                        Clinical Trials and submission of
                                        report to FDA

                     75,000             Completion of Phase III, Part B,
                                        Clinical Trials and submission of
                                        report to FDA

                    125,000             The earlier of NDA approval or
                                        contract termination, except in
                                        the event of a termination by WCCS
                                        under paragraphs 7.3, 7.4 or 7.7
                                        of this Agreement

          h.  REGISTRATION RIGHTS:   Subject to the approval of Entropin's
          investment banker, Entropin shall include the shares underlying
          options granted to WCCS in this Agreement in a registration
          statement filed with the Securities and Exchange Commission under
          the Act as soon as practicable following the completion of
          Entropin's proposed secondary public offering; provided, however,
          the shares underlying such options shall be subject to any lock-up
          or other restrictions imposed upon Entropin's directors,
          officers and major shareholders.  Entropin shall make all
          reasonable efforts to keep such registration current. Entropin
          shall give 30 days' prior written notice to WCCS of Entropin's
          intention to file a registration statement under the Act.  WCCS
          will provide Entropin with such information relating to WCCS and
          its plan of distribution as may be necessary for the completion
          of the

                                    4

<PAGE>

          registration statement, and will enter into appropriate
          indemnification agreements, as well as any lock-up and other
          restrictive agreements as entered into by principal shareholders,
          officers or directors of Entropin, and make other customary
          representations to Entropin related to the registration
          statement.

     4.   Article V of the Agreement entitled "ADJUSTMENTS" is hereby
amended as follows:

          5.1  INABILITY TO MEET CURRENT OBLIGATIONS:   In the event
          Entropin's financial forecasts project that Entropin is or will
          become unable to meet its financial obligations accrued in the
          normal course of business as they become due 60 days from the
          forecast date, the parties will enter good faith negotiations to
          modify this Agreement within thirty (30) days from written notice
          to WCCS by Entropin of such forecast.  If the parties are not
          able to negotiate an acceptable modification of the Agreement,
          either party may terminate the Agreement.  Notwithstanding
          paragraph 5.1(a), any negotiated reduction in WCCS' compensation
          will be accrued and paid by Entropin as soon as practicable,
          provided however, the parties reach an agreement on adjustments
          in performance. Such termination shall release all parties of all
          further obligations under the Agreement, except that Entropin
          shall have the following obligations to WCCS:   [PARAGRAPH 5.1(a)
          REMAINS UNCHANGED AND PARAGRAPH 5.1(b) IS ELIMINATED.]

     5.   Article VI of the Agreement entitled "DIRECTORS AND OFFICERS
LIABILITY INSURANCE" is hereby amended to include the following provision:

                                    5

<PAGE>

          6.3  INDEMNIFICATION: Entropin agrees to indemnify, defend, and
               hold harmless WCCS, its employees, subcontractors or agents,
               for any and all claims, demands and expenses, including
               litigation costs, arising out of, or in any way resulting
               from or connected with the services or work provided by
               WCCS, its employees, subcontractors or agents, insofar as
               such claim, liability or action arises out of, or is based
               upon, the actions or inactions of Entropin.

     5.   Article VII of the Agreement entitled "TERMINATION" is hereby
amended to include the following provisions:

          7.7  CONTINUATION OF THE AGREEMENT: In the event that the results
          of the Phase III, Part A, Clinical Trials indicate that
          Esterom(R) is ineffective, either party may terminate this
          Agreement upon written notice to the other party.

          7.8  OPTION TO TERMINATE: At any time after the completion of the
          Phase III, Part A, Clinical Trials, if Entropin shall have
          obtained a commitment with an alliance partner or an equivalent
          business relationship, Entropin shall have the option to
          terminate this Agreement effective upon sixty (60) days written
          notice to terminate.  At the termination date, all options
          granted to WCCS or its assignees under this Agreement shall
          become immediately exercisable.

     6.   Article XI of the Agreement entitled "ARBITRATION" is hereby
amended as follows:

          Any dispute arising between WCCS and Entropin concerning this
     agreement or its interpretation shall be resolved by submission to
     arbitration in Denver, Colorado in accordance with the rules of the
     American Arbitration Association then in effect.  The

                                    6

<PAGE>

     dispute will be heard by a panel of arbitrators familiar with
     pharmaceutical manufacturing and/or pharmaceutical research and
     development. Any award made by such arbitrators shall be binding and
     conclusive for all purposes hereof, may include injunctive relief, and
     may be entered as a final judgment in any court of competent
     jurisdiction.  The costs and expenses of such arbitration shall be
     borne in accordance with the determination of the arbitrators.

     7.   Article XII of the Agreement entitled "MISCELLANEOUS" is hereby
amended as follows:

          12.1 (a) if the WCCS:   Lois Rezler, Ph.D
                                  Western Center for Clinical Studies, Inc.
                                  21550 Oxnard Street, Suite 810
                                  Woodland Hills, CA 91367

     8.   The following Exhibits shall be amended as attached hereto as
Exhibits A and D, respectively:

          Exhibit A: BUDGET
          -----------------

          Exhibit D: TIME LINE: Milestones, Tasks, and Sub Tasks
          ------------------------------------------------------

     All other provisions of the Agreement shall remain in full force and
effect.

     IN WITNESS WHEREOF, the undersigned have entered into this AMENDMENT
effective July 21, 1999.

WESTERN CENTER FOR CLINICAL
   STUDIES, INC.                        ENTROPIN, INC.

By:  /s/ LOIS REZLER                    By:  /s/ DONALD HUNTER
     -----------------------------           -----------------------------
     Lois Rezler                             Donald Hunter
     President                               Chief Executive Officer

Date: 7/28/99                           Date:   7/21/99
     -----------------                       -----------------

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