<PAGE>

                                                                    Exhibit 10.3

                             CO-OWNERSHIP AGREEMENT

      THIS CO-OWNERSHIP AGREEMENT (the "Agreement") is by and between PHARSIGHT
CORPORATION, a California corporation having its principal place of business at
299 California Avenue, Suite 300, Palo Alto, CA. 94306 ("Pharsight") and
MITCHELL AND GAUTHIER ASSOCIATES, INC., a Delaware corporation having its
principal place of business at 200 Baker Avenue, Concord, Massachusetts 01742
("MGA"). This Agreement is effective as of May 27, 1998 (the "Effective Date").
Except as specified herein, all the capitalized terms used in this Agreement
shall retain the same meaning as defined in the Asset Purchase Agreement.

                                    RECITALS

      A. MGA owns certain computer software programs that are based on the
Advanced Continuous Simulation Language; and

      B. MGA is willing to assign a fifty percent (50%) undivided interest in
certain of these software programs to Pharsight in conjunction with the
execution of the Asset Purchase Agreement to be entered into among Pharsight,
MGA, Edward E. L. Mitchell and Joseph S. Gauthier on this date (the "Asset
Purchase Agreement").

                                    AGREEMENT

      The parties, intending to be legally bound, hereby agree as follows:

1. DEFINITIONS.

      1.1 "ACSL Products" shall mean the object code version of the products
which are listed and described in Exhibit A.

      1.2 "Derivative Work" shall mean a work which is based on the Software,
such as a revision, enhancement, modification, translation, abridgement,
condensation, expansion, or any other form in which the Software may be recast,
transformed, or adapted, and which, if prepared without authorization of the
owner of the copyright in the Software, would constitute a copyright
infringement. For purposes hereof, Derivative Work shall also include any
compilation that incorporates the Software.

      1.3 "Golden Master" shall mean, with respect to a given software program,
program module, or documentation, a copy of such item that (i) is under source
code control, (ii) is complete and accurate, (iii) represents the producer's
best efforts to meet the applicable specifications, (iv) has passed all
applicable test procedures, and (v) complies with normal industry practices for
verification and duplication.

      1.4 "Intellectual Property" means any or all of the following and all
statutory and/or common law rights throughout the world in, arising out of, or
associated therewith: (i) all patents and applications therefor and all
reissues, divisions, renewals, extensions, provisionals, continuations and
continuations-in-part thereof; (ii) all inventions (whether patentable or not),
invention disclosures and improvements, all trade secrets, proprietary
information, know-how


                                       1.
<PAGE>

and technology; (iii) all works of authorship, "moral rights", copyrights
(including derivative works thereof), mask works, copyright and mask work
registrations and applications; (iv) all industrial designs and any
registrations and applications therefor; (v) all trade names, logos, trademarks
and service marks, trademark and service mark registrations and applications
together with the good will of the business symbolized by the names and the
marks; (vi) all computer software and related documentation including all Source
Code, Object Code, firmware, installation programs, source code control
archives, development tools, test plans, test files, test data, build scripts,
file specifications, design and implementation documents, interface
specifications, requirements specifications, data bases, utilities, bug
databases, and all media on which any of the foregoing is recorded; (vii) any
similar, corresponding or equivalent rights to any of the foregoing; and (viii)
all goodwill associated with any of the foregoing.

      1.5 "Intellectual Property Rights" shall mean any interest or right
protectable under the law of any country as to any form of Intellectual
Property.

      1.6 "MGA Fields" shall mean all fields other than the Pharsight Fields.

      1.7 "Object Code" shall mean the machine-executable object code form of
the Software.

      1.8 "Pharsight Fields" shall mean the biotechnology, pharmaceutical
(including over-the-counter), veterinary, medical devices, environmental and
healthcare fields.

      1.9 "Software" shall mean the computer programs which are listed and
described in Exhibit A, including any upgrades, updates, works in progress and
all associated documentation (existing as of the Effective Date), developed by
or for MGA and any other components, programs or utilities required for the
operation or execution thereof, except those identified in Exhibit D. Unless
otherwise specified, software shall include both Source Code and Object Code.

      1.10 "Source Code" shall mean the human-readable source code version of
the Software and all related program documentation such that a programmer
reasonably skilled in the programming language could read, understand and modify
the Software.

2. ASSIGNMENT OF FIFTY PERCENT UNDIVIDED INTEREST.

      2.1 Assignment. MGA hereby irrevocably assigns to Pharsight a fifty
percent (50%) undivided right, title and interest worldwide in and to the
Software including, without limitation, patents, copyrights, trade secrets and
any other Intellectual Property Rights (excluding any related trademarks and
trade names) incorporated in the Software, whether in the United States or
abroad. Pharsight acknowledges that MGA has not filed any patent applications
nor obtained any issued patents on the Software as of the Effective Date. It is
the intention of the parties that the Intellectual Property Rights (excluding
any related trademarks and trade names) in the Software shall be jointly owned
without any duty to account or share any royalties based on the licensing or use
of the Software by the other party and as permitted under Section 5.

      2.2 Waiver or Assignment of Other Rights. If MGA has any rights in or to
the Software in which a fifty percent (50%) interest cannot be assigned to
Pharsight, MGA


                                       2.
<PAGE>

unconditionally and irrevocably waives the enforcement of such rights, and all
claims and causes of action of any kind against Pharsight with respect to such
rights, and agrees, at Pharsight's request and expense, to consent to and join
in any action to enforce such rights. If MGA has any rights in or to the
Software that cannot be assigned to Pharsight or waived by MGA, MGA
unconditionally and irrevocably grants to Pharsight during the term of such
rights, an irrevocable, perpetual, worldwide, fully paid and royalty-free
license, with rights to sublicense through multiple levels of sublicensees, to
reproduce, create derivative works of, distribute, publicly perform and publicly
display by all means now known or later developed, such rights in or to the
Software.

      2.3 Further Assistance. MGA agrees to cooperate with Pharsight or its
designee(s) in the procurement and maintenance of Pharsight's rights in the
Software and to execute, when requested, any other documents deemed necessary by
Pharsight to carry out the purpose of this Agreement. MGA agrees to execute a
signed transfer of a fifty percent (50%) undivided interest of copyright to
Pharsight in the form attached to this Agreement as Exhibit B for all Software
and related documentation.

      2.4 Enforcement of Intellectual Property Rights. Upon reasonable request,
each party (the "Assisting Party") will assist the other party (the "Requesting
Party") in every proper way to obtain and from time to time enforce, United
States and foreign Intellectual Property Rights related to the Software in any
and all countries. To that end, the Assisting Party will execute, verify and
deliver such documents and perform such other acts (including appearances as a
witness) as the Requesting Party may reasonably request for use in applying for,
obtaining, perfecting, evidencing, sustaining and enforcing such Intellectual
Property Rights and the ownership of a fifty percent (50%) undivided interest
thereof. The Assisting Party's obligation to assist the Requesting Party with
respect to such Intellectual Property Rights relating to the Software in any and
all countries shall continue in perpetuity, but the Requesting Party shall
compensate the Assisting Party at a reasonable rate for the time actually spent
by such party in providing such assistance. In addition, the Assisting Party
will execute any license agreement requiring the names of both co-owners to make
the license enforceable.

      2.5 Execution of Documents. In the event Pharsight is unable for any
reason, after reasonable effort, to secure MGA's signature on any document
needed in connection with the actions specified in Section 2.3 above, MGA hereby
irrevocably designates and appoints Pharsight and its duly authorized officers
and agents as its agent and attorney in fact, which appointment is coupled with
an interest, to act for and in its behalf to execute, verify and file any such
documents and to do all other lawfully permitted acts to further the purposes of
the preceding paragraph with the same legal force and effect as if executed by
MGA. Also, in the event the Requesting Party is unable for any reason, after
reasonable effort, to secure the Assisting Party's signature on any document
needed in connection with the actions specified in Section 2.4 above, the
Assisting Party hereby irrevocably designates and appoints the Requesting Party
and its duly authorized officers and agents as its agent and attorney in fact,
which appointment is coupled with an interest, to act for and in its behalf to
execute, verify and file any such documents and to do all other lawfully
permitted acts to further the purposes of the preceding paragraph with the same
legal force and effect as if executed by the Assisting Party.


                                       3.
<PAGE>

      2.6 Copyright Notice. MGA and Pharsight agree that all copies of the
Software and any Derivative Works, whether or not modified, made by MGA or
Pharsight, or their respective licensees or sublicensees, will contain
applicable proprietary notices. In addition, MGA and Pharsight agree to include
an applicable copyright notice in or on the media of all copies of the Software
or any Derivative Work.

      2.7 Patent Applications. Notwithstanding the provisions of Section 7,
either party (the "Proposing Party") may prepare and file a patent application
for a Derivative Work in any country; provided that the Proposing Party (i)
notifies the other party (the "Co-Owner"), in writing, at least sixty (60) days
prior to submitting such patent application; (ii) describes the scope of the
proposed patent application and the countries in which it desires to seek patent
protection; and (iii) permits the Co-Owner to participate, at Co-Owner's
expense, in the prosecution of the patent. The ownership of the patent in a
particular country shall be proportional to the parties' contributions for the
payment of the patent prosecution and maintenance fees. In the event that the
Co-Owner elects not to participate in the patent prosecution, the Co-Owner shall
provide information reasonably necessary for the Proposing Party to prosecute
such patent applications, at the expense of the Proposing Party. If either party
("Abandoning Party") desires to abandon a patent application or a patent issued
thereto, such party shall provide the other party ("Objecting Party") with sixty
(60) days' prior written notice of its intent to abandon such application or
patent. If the Objecting Party desires to continue such prosecution or
maintenance of the patent, the Abandoning Party shall assign all right, title
and interest in and to the application and/or the issued patent to the Objecting
Party and the Objecting Party shall be the sole owner of the application and/or
the issued patent. Notwithstanding any of the foregoing, in no event shall any
issued patent, whether owned solely or jointly, preclude either party from
exercising its rights, exclusive or otherwise, in or to the Software as set
forth in this Agreement.

3. DELIVERY. Upon execution of this Agreement, MGA shall provide to Pharsight
five (5) Golden Masters of the Object Code, two (2) electronic copies of the
Source Code corresponding to the Golden Master Object Code, and five (5) Golden
Masters and two (2) printed copies of any end-user documentation for all
programs included in the Software. Without limiting the foregoing, MGA's
delivery shall include everything Pharsight needs to recreate the most current
released version, and every supported prior version, of each of the programs
included in the software for all currently supported platforms, including copies
of all firmware, installation programs, source code control archives,
development tools, test plans, test files, test data, build scripts, file
specifications, design and implementation documents, interface specifications,
requirements specifications, data bases, utilities, and bug databases.

4. LICENSE GRANTS.

      4.1 MGA Grant to Pharsight. Subject to the terms and conditions of this
Agreement and except for the termination periods of the distribution agreements
referenced in Section 2.4 (a) (viii) of the Asset Purchase Agreement, MGA hereby
grants to Pharsight an exclusive (even as to MGA), perpetual, irrevocable,
worldwide, royalty-free license to use, reproduce, create Derivative Works of,
distribute, sublicense (through multiple tiers of sublicensees), perform and
publicly display the Software in the Pharsight Fields.


                                       4.
<PAGE>

      4.2 Pharsight Grant to MGA. Subject to the terms and conditions of this
Agreement, Pharsight hereby grants to MGA an exclusive (even as to Pharsight,
except as set forth in this provision), perpetual, irrevocable, worldwide,
royalty-free license to use, reproduce, create Derivative Works of, distribute,
sublicense (through multiple tiers of sublicensees), perform and publicly
display the Software in the MGA Fields. Notwithstanding the foregoing, Pharsight
hereby expressly reserves the right to use, reproduce, create Derivative Works
of, distribute, sublicense (through multiple tiers of sublicensees), perform and
publicly display the Software, and any Derivative Works thereof, in the MGA
Fields, incorporated in or bundled with an additional Pharsight product
(including, without limitation, ACSL BioMed or ACSL Tox) so long as the
resultant product (i) is designed for use in the Pharsight Fields and (ii)
represents an enhancement and/or transformation of the Software (with regard to
both value and function).

      4.3 Trademark License. MGA hereby grants to Pharsight an unrestricted,
perpetual, irrevocable, exclusive (even as to MGA), royalty-free, world-wide
license to use all trademarks and trade names owned by MGA and associated with
the Software (herein the "Trademarks") in conjunction with any product designed
for use in the Pharsight Fields. Pharsight agrees to state in appropriate places
that the Trademarks are the trademarks of MGA and to include the symbols (TM)
and (R) as appropriate. Pharsight may grant sublicenses of its rights to use the
Trademarks provided such sublicensees have executed written agreements with
Pharsight containing terms and conditions substantially similar to those
contained herein. Pharsight agrees to maintain the quality of the software
products licensed using the Trademarks, to use the Trademarks in accordance with
guidelines and instructions as may be reasonably promulgated by MGA from time to
time, and to provide to MGA, upon reasonable request but no more often than once
per year, representative samples of product packaging using the Trademarks. MGA
agrees to take all actions reasonably necessary to protect Pharsight's rights in
the Trademarks including, without limitation: (i) maintaining all required
registrations; and (ii) promptly notifying Pharsight of any adverse use by a
third party of the Trademarks or of a mark or a name confusingly similar to the
Trademarks.

5. LICENSING OF THE SOFTWARE.

      5.1 MGA Object Code Licenses. Pharsight acknowledges and agrees that MGA
may license the Object Code and/or MGA's Derivative Works in Object Code format
(herein "MGA Object Code") to third parties in accordance with its standard
licensing practices; provided that such licenses are granted only to third
parties in fields other than the Pharsight Fields and include prohibitions on
obtaining the Source Code through reverse engineering.

            (a) Such MGA Object Code licenses may include a Source Code escrow
provision; provided that (i) a reputable Source Code escrow agency will maintain
the Source Code or the Source Code of the Derivative Work; (ii) the license to
use the Source Code or the Source Code of the Derivative Work in the event the
licensee rightfully receives the Source Code from the escrow agent shall be
restricted to fields other than the Pharsight Fields; and (iii) MGA agrees to
promptly notify Pharsight of any release of the Source Code or the Source Code
of the Derivative Work from the escrow account.


                                       5.
<PAGE>

            (b) To the extent necessary, such MGA Object Code licenses may
include provisions requiring MGA to provide the third party with interpreted
code for models, libraries, etc.

      5.2 Pharsight Object Code Licenses. MGA acknowledges and agrees that
Pharsight may license the Object Code and/or Pharsight's Derivative Works in
Object Code format (herein Pharsight Object Code") to third parties in
accordance with its standard licensing practices; provided that such licenses
are granted only to third parties in the fields other than the MGA Fields and
include prohibitions on obtaining the Source Code through reverse engineering.

            (a) Such Pharsight Object Code licenses may include a Source Code
escrow provision; provided that (i) a reputable Source Code escrow agency will
maintain the Source Code or the Source Code of the Derivative Work; (ii) the
license to use the Source Code or the Source Code of the Derivative Work in the
event the licensee rightfully receives the Source Code from the escrow agent
shall be restricted to fields other than the MGA Fields; and (iii) Pharsight
agrees to promptly notify MGA of any release of the Source Code or the Source
Code of the Derivative Work from the escrow account.

            (b) To the extent necessary, such Pharsight Object Code licenses may
include provisions requiring Pharsight to provide the third party with
interpreted code for models, libraries, etc.

      5.3 MGA Source Code Licenses. MGA acknowledges and agrees that in order to
maintain the economic value of the Software, any license entered into by MGA of
all or any portion of the Source Code or of any Derivative Work in Source Code
format (herein "MGA Source Code") shall be restricted as follows:

            (a) All such licenses shall be in writing and executed by the third
party and shall include, at a minimum, provisions substantially similar to those
contained in Exhibit C;

            (b) Pharsight shall be named as a third party beneficiary of such
licenses;

            (c) The third party shall agree to use the MGA Source Code solely
for its internal purposes and agree to maintain written records of the location
of each copy of the MGA Source Code and permit audit of such records by MGA and
Pharsight;

            (d) The licenses shall restrict the use of the MGA Source Code, and
any derivative works created by or for the third party, to the MGA Fields;

            (e) MGA shall promptly send a copy of all such licenses to
Pharsight, although MGA may delete information relating to license fees and
other financial terms from such copies; and

            (f) MGA agrees to report any violations by any third party of the
confidentiality, notice and other provisions that are important to maintain the
value of the Intellectual Property Rights in the Software and to assist
Pharsight in enforcing such provisions.


                                       6.
<PAGE>

      5.4 Pharsight Source Code Licenses. Pharsight acknowledges and agrees that
in order to maintain the economic value of the Software, any license entered
into by Pharsight of all or any portion of the Source Code or of any Derivative
Work in Source Code format (herein "Pharsight Source Code"), shall be restricted
as follows:

            (a) All such licenses shall be in writing and executed by the third
party and shall include, at a minimum, provisions substantially similar to those
contained in Exhibit C;

            (b) MGA shall be named as a third party beneficiary of such
licenses;

            (c) The third party shall agree to use the Pharsight Source Code
solely for its internal purposes and agree to maintain written records of the
location of each copy of the Pharsight Source Code and permit audit of such
records by Pharsight and MGA;

            (d) The licenses shall restrict the use of the Pharsight Source
Code, and any derivative works created by or for the third party, to the
Pharsight Fields;

            (e) Pharsight shall promptly send a copy of all such licenses to
MGA, although Pharsight may delete information relating to license fees and
other financial terms from such copies; and

            (f) Pharsight agrees to report any violations by any third party of
the confidentiality, notice and other provisions that are important to maintain
the value of the Intellectual Property Rights in the Software and to assist MGA
in enforcing such provisions.

      5.5 No Other Transfer of Right, Title or Interest. Except as set forth in
Sections 5.1, 5.2, 5.3, 5.4 and 13.2, neither party shall grant any right, title
or interest in or to the Software or any Derivative Work without the prior
written consent of the other party, which consent shall not be unreasonably
withheld.

6. OWNERSHIP OF DERIVATIVE WORKS. Upon execution of this Agreement, MGA and
Pharsight shall each own a fifty percent (50%) undivided interest in and to the
Software as it exists on the Effective Date. After the Effective Date, each
party shall be the sole and exclusive owner of any right, title and interest in
or to any Derivative Works of the Software, including any Intellectual Property
Rights contained therein, created by or for such party, other than the original
Software.

7. CONFIDENTIALITY. The parties acknowledge and agree that the Source Code
contains valuable trade secrets and each party shall take all reasonable steps
to prevent unauthorized disclosure or use of the Source Code and to prevent it
from falling into the public domain (other than pursuant to a patent filing
related to a Derivative Work made in accordance with Section 2.7) or into the
possession of unauthorized persons. Except as provided in Sections 5.3 and 5.4
of this Agreement, neither party shall disclose the Source Code or any
Derivative Work in Source Code form, in whole or in part, to any person or
entity other than its officers, employees or contractors that have entered into
written confidentiality agreements with such party that protect the
confidentiality of the Source Code and/or the Source Code of the Derivative
Work. Each party shall give notice to the other party of any unauthorized use or
disclosure of the Source Code or Source Code of the Derivative Work and agrees
to assist the other party in


                                       7.
<PAGE>

remedying any such unauthorized use or disclosure. This obligation shall not
extend to any Intellectual Property information in the Software if such
information (i) has been published or is otherwise readily available to the
public other than by a breach of this Agreement; (ii) has been rightfully
received from a third party without confidential limitations; (iii) has been
independently developed for the party by personnel or agents having no access to
the Source Code or Source Code of the Derivative Work; or (iv) was known to
Pharsight prior to Pharsight's first receipt of such information from MGA.

8. INFRINGEMENT BY THIRD PARTIES.

      8.1 Prior Notice. The parties shall consult with each other prior to
filing any action alleging that a third party has infringed or misappropriated
any Intellectual Property Rights of the Software. The parties may agree to
jointly pay for the suit or otherwise share such costs and any resulting
liability or monetary judgment. Except as provided in Section 8.2, if no
agreement is reached within sixty (60) days, the party wishing to file such
action may do so, but shall pay the entire cost of such action and shall
indemnify and hold harmless the non-filing party from any claim, suit or
proceeding (including, without limitation, counterclaims) against such party
arising from the action brought by the filing party. If only one party decides
to proceed with an action and it prevails, that party shall be entitled to
retain the entire amount of any monetary award arising out of that action.

      8.2 Immediate Relief. If one party believes that immediate relief is
necessary to protect the trade secrets or other Intellectual Property Rights in
and to the Software, it may, upon written notice to the other party, immediately
file an action to protect such rights. Unless the parties agree otherwise, the
party filing such an immediate action shall have the same rights and obligations
as if no agreement was reached between the parties and one party proceeded to
file a suit as provided in Section 8.1 above.

      8.3 Indemnity. To be eligible for the indemnity under Section 8.1 above, a
party must give prompt written notice of any claim, suit or proceeding filed or
threatened against it and let the indemnifying party control the defense.

      8.4 Assistance. If only one party files an action as provided in Section
8.1 or 8.2 above, the other party agrees to assist in such action so long as the
filing party pays its out-of-pocket expenses.

9. REPRESENTATIONS, WARRANTIES AND COVENANTS. Each party hereby represents and
warrants that (i) it will not grant, directly or indirectly, any rights or
interest in the Software to third parties that are inconsistent with the rights
assigned to, or retained by, the other party herein; and (ii) it has the full
right and power to enter into and perform this Agreement without the consent of
any third parties.

10. INDEMNITY. Each party (the "Indemnifying Party") shall indemnify and hold
harmless the other party, its officers, directors, employees, sublicensees,
customers, contractors and agents (the "Indemnitees") from any and all claims,
losses, liabilities, damages, expenses and costs (including attorneys' fees and
court costs) which result from a breach or an alleged breach of any
representation or warranty by the Indemnifying Party identified in Section 9
("Representations,


                                       8.
<PAGE>

Warranties and Covenants") or any covenant by the Indemnifying Party set forth
in this Agreement.

11. SUPPORT OBLIGATIONS. In addition to the obligations set forth in this
Agreement, the parties hereby acknowledge and agree to provide the support
services set forth in the Support Appendix attached hereto.

12. TERM. Except as expressly provided herein, the terms of this Agreement shall
commence upon execution and continue in perpetuity.

13. GENERAL PROVISIONS.

      13.1 Governing Law.

            (a) This Agreement shall be construed in accordance with, and
governed in all respects by, the internal laws of the State of California
(without giving effect to principles of conflicts of laws).

            (b) Any dispute, claim or controversy of any nature arising out of
or relating to this Agreement, including without limitation any action or claim
based on tort, contract, statute, or for any other cause of action, and which
relates in any way to the interpretation, effect, termination, validity,
enforcement, performance and/or breach of this Agreement, shall be resolved by
final binding arbitration administered by the American Arbitration Association
("AAA"). The arbitration shall be conducted before a panel of three arbitrators
under the commercial arbitration rules of the AAA and shall be held at an AAA
facility (i) in Boston, Massachusetts, if brought by Buyer, and (ii) in Santa
Clara County, California, if brought by Sellers. The parties hereto agree that
all arbitrators serving on such panel must be available to serve on the panel in
accordance with the timetable of the arbitration.

            (c) Not for the adjudication of any matters (other than judicial
review for fraud or undisclosed bias), but for the enforcement of an arbitration
award (which shall be brought in California if Buyer is enforcing the
arbitration award or in Massachusetts if one or more Sellers are enforcing the
arbitration award) or the granting of injunctive relief (which shall be brought
in Massachusetts if Buyer is seeking injunctive relief or in California if one
or more Sellers are seeking injunctive relief), the parties hereto irrevocably
elect as the sole two judicial forums for the adjudication of any matters
arising under or in connection with this Agreement, and consent to the
jurisdiction of, the courts of the State of California and the Commonwealth of
Massachusetts.

      13.2 No Assignment. This Agreement shall be binding upon and inure to the
benefit of the parties named herein and their respective successors and
permitted assigns. Neither this Agreement nor any rights or obligations
hereunder may be assigned by either party in whole or in part without the prior
written consent of the other party. Notwithstanding the foregoing, either party
may assign this Agreement to an entity that acquires all or substantially all of
the assets of such party; provided that such permitted assignee agrees to be
bound by the terms hereof and to assume all obligations of the assigning party
hereunder.


                                       9.
<PAGE>

      13.3 Headings. The section headings contained in this Agreement are
inserted for convenience only and shall not affect in any way the meaning or
interpretation of this Agreement.

      13.4 Severability. In case any one or more of the provisions contained in
this Agreement shall, for any reason, be held to be invalid, illegal or
unenforceable in any respect, such invalidity, illegality or unenforceability
shall not affect the other provisions of this Agreement, and this Agreement
shall be construed as if such invalid, illegal or unenforceable provision had
never been contained herein. If moreover, any one or more of the provisions
contained in this Agreement shall for any reason be held to be excessively broad
as to duration, geographical scope, activity or subject, it shall be construed
by limiting and reducing it, so as to be enforceable to the extent compatible
with the applicable law as it shall then appear.

      13.5 Notices. Any notice or other communication required or permitted to
be delivered to any party under this Agreement shall be in writing and shall be
deemed properly delivered, given and received (i) when delivered by hand, (ii)
five days after deposit in the United States mail if by certified mail, (iii) on
the next business day if sent by courier or express delivery service that
guarantees next business day delivery, or (iv) upon confirmation if sent by
facsimile, to the address or facsimile telephone number set forth beneath the
name of such party below (or to such other address or facsimile telephone number
as such party shall have specified in a written notice given to the other
parties hereto):

            If to MGA:

                  MITCHELL AND GAUTHIER ASSOCIATES, INC.
                  919 B Willowbrook Drive
                  Huntsville, Alabama  35802
                  Attention: President
                  Facsimile: (205) 883-5516

            Copy to:

                  Richard Stein
                  Hutchins, Wheeler & Dittmar
                  101 Federal Street
                  Boston, MA 02110
                  Facsimile: (617) 951-1295

            If to Pharsight:

                  PHARSIGHT CORPORATION
                  299 California Ave.
                  Suite 300
                  Palo Alto, CA 94306
                  Attention: Arthur H. Reidel, President
                  Facsimile: (650) 462-5610


                                      10.
<PAGE>

            Copy to:

                  Cooley Godward LLP
                  5 Palo Alto Square
                  Palo Alto, California 94306
                  Attention: Andrei M. Manoliu, Esq.
                  Facsimile: (650) 857-0663

      13.6 Legal Fees. If any dispute arises between the parties with respect to
the matters covered by this Agreement which leads to a proceeding to resolve
such dispute, the prevailing party in such proceeding shall be entitled to
receive its reasonable attorneys' fees, expert witness fees and out-of-pocket
costs incurred in connection with such proceeding, in addition to any other
relief it may be awarded.

      13.7 Injunctive Relief. A breach of any of the promises or agreements
contained in this Agreement by one party may result in irreparable and
continuing damage to the other party for which there may be no adequate remedy
at law, and the non-breaching party is therefore entitled to seek injunctive
relief as well as such other and further relief as may be appropriate.

      13.8 Waiver. No waiver by either party of any breach of this Agreement
shall be a waiver of any preceding or succeeding breach. No waiver by either
party of any right under this Agreement shall be construed as a waiver of any
other right. The parties shall not be required to give notice to enforce strict
adherence to all terms of this Agreement.

      13.9 Counterparts. This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument.

      13.10 Entire Agreement. This Agreement, including the Exhibits and
Appendix attached hereto, together with the Asset Purchase Agreement, is the
final, complete and exclusive agreement of the parties with respect to the
subject matter hereof and supersedes and merges all prior discussions between
us. No modification of or amendment to this Agreement, nor any waiver of any
rights under this Agreement, will be effective unless in writing and signed by
the party to be charged.

      IN WITNESS WHEREOF, the parties have caused this Co-Ownership Agreement to
be executed by their duly authorized representative.

PHARSIGHT CORPORATION:

    /s/ Arthur H. Reidel
---------------------------------------
        Arthur H. Reidel
---------------------------------------
        (Printed Name)

By:     Arthur H. Reidel
    -----------------------------------
Title:     President
    -----------------------------------

MITCHELL AND GAUTHIER ASSOCIATES, INC.:
    /s/ Michael Gauthier
---------------------------------------
        Michael Gauthier
---------------------------------------
        (Printed Name)

By:     Michael Gauthier
    -----------------------------------
Title:     illegible
    -----------------------------------


                                      11.
<PAGE>

                                    EXHIBIT A

                             ACSL SOFTWARE PROGRAMS

Existing products:

ACSL: A translator program, a system macro file, a run time library, a builder
program, a run time front end program, programs/procedures to install/uninstall
all of the preceding on all supported computer systems.

ACSL GM: A graphic modeling program, standard racks, programs/procedures to
install/uninstall all of the preceding on all supported computer systems.

ACSL Math: An analysis and visualization program, directories of standard
m-files, programs/procedures to install/uninstall all of the preceding on all
supported compute systems.

ACSL Optimize: The optimization extension of ACSL Math.

ACSL Viewer: The viewer extension of ACSL GM.

ACSL Model: A combination of ACSL, ACSL GM and ACSL MATH, bundled together for
marketing purposes.

ACSL Server: An interface to the model.prx file (the .dll file) that represents
the user model so that it can be driven and interrogated by an external program
such as Visual Basic or any other OLE driver. Only works on a PC.

ACSL Vision: This is ACSL bundled with a DataViews product that really only runs
on a Unix/X Windows system.

ACSL Real Time: A real-time system for a PC version that allows the user to
simulate plants (physical systems represented by differential equations) and
test out hardware controllers on these plants using a real-time clock.


                                       1.
<PAGE>

                                    EXHIBIT B

                             ASSIGNMENT OF COPYRIGHT

      For good and valuable consideration which has been received, the
undersigned sells, assigns and transfers to Pharsight, a California corporation,
and its successors and assigns, a fifty percent (50%) undivided interest in the
copyright in and to the following work, which was created by the following
indicated author(s):

Title: ________________________________

Author(s): ____________________________

           ____________________________

Copyright Office Identification No. (if any): _________________________

and all of the right, title and interest of the undersigned, vested and
contingent, therein and thereto.

      Executed this ________________ day of _________ , 1998.


                              Signature: _______________________________________

                              Printed Name: ____________________________________


                                       1.
<PAGE>

                                    EXHIBIT C

                          MINIMUM TERMS AND CONDITIONS

1. Ownership. The Software is licensed, not sold, to Licensee for use only under
the terms of this Agreement, and Licensor and its Co-owner reserve all rights
not expressly granted to Licensee. Licensee owns the media, if any, on which the
Software is recorded, but Licensor and its Co-owner retain ownership of all
copies of the Software itself.

2. Reservation of Rights. Except as stated above, this Agreement does not grant
Licensee any intellectual property rights in the Software.

3. Termination. Licensor shall have the right to terminate this Agreement in the
event Licensee materially breaches any term or condition herein. Licensee agrees
upon termination to promptly destroy the Software and all copies.


                                       1.
<PAGE>

                                    EXHIBIT D

                               SOFTWARE EXCEPTIONS

Watcom Fortran from Sybase Corporation (formerly Powersoft)

GRG2 Software from Windward Technology

FlexLM Software from Globetrotter (formerly Highland Software)

MainWin Software from

Mainsoft


                                       1.
<PAGE>

                                SUPPORT APPENDIX

      This Support Appendix sets forth the terms and conditions pursuant to
which MGA and Pharsight will provide support and training services to the other
party related to the MGA Restricted Software and shall be incorporated by
reference into the Co-Ownership Agreement. Such terms and conditions are
applicable only to the support and training services described below. All
capitalized terms used in this Appendix shall retain the same meaning as defined
in the Co-Ownership Agreement and/or the Asset Purchase Agreement.

1. TYPES OF SUPPORT

      1.1 Mutual Support.

            a. Support and Training. Each party will provide to the other party
support and training related to the MGA Restricted Software.

            b. Ownership. Any new inventions, discoveries or developments
jointly conceived by the parties resulting from the support or training provided
under this Support Appendix shall be "co-owned" by the parties pursuant to the
terms and conditions of the Co-Ownership Agreement.

            c. Travel and Expenses. Travel and expenses, if any, shall be
pre-approved and paid by the party requesting the support or training.

      1.2 MGA Transition Support. Pharsight will provide limited assistance to
MGA for transitioning MGA's business to Huntsville, Alabama. Such assistance may
include the following:

            a. Assistance with moving, copying and storing development tools and
programs.

            b. Assistance with electronic file transmissions.

            c. Assistance transitioning MGA's website, mail server, media
duplication and information technology.

            d. Assistance shipping computers to Huntsville, Alabama and support
in re-activating them

      1.3 MGA Unix and VAX ACSL Support. MGA shall provide to Pharsight
"support and maintenance" for existing customers in the Pharsight Fields using
Unix and VAX-based ACSL software. "Support and Maintenance" means (a)
second-tier customer support; and (b) assisting Pharsight developers with bug
fixes and resolving technical questions.

            a. MGA shall provide second-tier customer support to Pharsight
consisting of the following:

                  (i) MGA to ensure a maximum of four hour initial response on
incoming technical support-related calls during normal business hours.


                                       1.
<PAGE>

                  (ii) MGA to respond to email support questions within 24
hours, excluding weekends and holidays.

            b. New releases, upgrades and/or bug fixes provided by MGA shall be
subject to the indemnification provisions of the Asset Purchase Agreement for
3rd party claims based upon infringement.

2. LIMITATIONS ON SUPPORT AND TRAINING.

      2.1 Support. Support provided by one party to the other during the first
3 months following the Closing shall not exceed 80 person-hours and thereafter
shall not exceed 15 person-hours per month or a total of 225 person-hours over
the term of this Support Appendix. Additional support in excess of these amounts
may be provided, at the discretion of the providing party, at such party's
then-current support rates.

      2.2 Training. Pharsight employees Sining Fang, Mike Dunlavey and Ken
Busch will be available, upon reasonable request, for a combined total of 90
hours to provide training to MGA related to the MGA Restricted Software during
the first 6 months following the Closing. A comparable number of hours actually
used by MGA for such training shall be provided by MGA to Pharsight for
additional support and/or training upon reasonable request.

3. TERM AND TERMINATION.

      3.1 Term. The obligations of this Support Appendix shall commence on date
of closing and continue for a period of two (2) years.

      3.2 Termination for Breach. Either party may terminate this Support
Appendix upon thirty (30) days' written notice of a material breach of this
Support Appendix if such breach is not cured within such thirty (30) day period.

4. WARRANTY DISCLAIMER. EXCEPT AS EXPRESSLY PROVIDED HEREIN, THE SUPPORT AND
TRAINING SERVICES PROVIDED IN THIS SUPPORT APPENDIX ARE PROVIDED ON AN "AS IS"
BASIS WITHOUT ANY WARRANTY WHATSOEVER, AND EACH PARTY EXPRESSLY DISCLAIMS ALL
WARRANTIES, EXPRESS, IMPLIED, AND STATUTORY INCLUDING WITHOUT LIMITATION THE
IMPLIED WARRANTIES OF MERCHANTABILITY, TITLE, NON-INFRINGEMENT OF THIRD PARTY
RIGHTS AND FITNESS FOR A PARTICULAR PURPOSE.

5. LIMITATION OF LIABILITY. NEITHER PARTY SHALL BE LIABLE TO THE OTHER FOR ANY
LOSS OF USE, INTERRUPTION OF BUSINESS OR ANY SPECIAL, INDIRECT, INCIDENTAL OR
CONSEQUENTIAL DAMAGES OF ANY KIND (INCLUDING LOST PROFITS) REGARDLESS OF THE
FORM OF ACTION WHETHER IN CONTRACT, TORT (INCLUDING NEGLIGENCE), STRICT
LIABILITY OR OTHERWISE RELATED TO THE PERFORMANCE OF THIS SUPPORT APPENDIX, EVEN
IF SUCH PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.


                                       2.