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                                                                   Exhibit 10.23


                      CHANGE OF CONTROL/SEVERANCE AGREEMENT
                      -------------------------------------

     This CHANGE OF CONTROL/SEVERANCE AGREEMENT, dated as of April 3, 2001 by
and between PAREXEL International Corporation (together with all subsidiaries or
affiliates hereinafter referred to as the "Company") and James F. Winschel, Jr.
(the "Executive").

     WHEREAS, the Executive has been hired as a senior executive of the Company
and is expected to make major contributions to the Company;

     WHEREAS, the Company desires continuity of management; and

     WHEREAS, the Executive is willing to render services to the Company subject
to the conditions set forth in this Agreement.

     NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the Company and the Executive agree
as follows:

     1. Termination without Cause. In the event the Company terminates the
        -------------------------
Executive's employment with the Company without "Cause" (as such term is defined
in Section 4(c) below), the Company shall pay to the Executive, within ten
business days following the date of termination, a lump sum amount (net of any
required withholding) equal to: (i) twelve (12) months of monthly base salary
(at the highest monthly base salary rate in effect for the Executive in the
twelve month period prior to the termination of his employment), plus (ii) the
pro rata share of the target bonus that could have been payable to the Executive
pursuant to the Company's Performance Bonus Plan (assuming continued employment)
during the year in which the termination occurs, based on bonus arrangements in
effect at any time during the twelve month period immediately prior to the
termination of his employment, such pro rata share to be calculated from the
beginning of the fiscal year in which the termination occurs through the date of
termination.

     2. Termination Prior to a Change of Control.
        ----------------------------------------

     (a) Notwithstanding the provisions of Section 1 above, if, within nine
months prior to a Change of Control (as such term is defined in Section 4(b)
below) and subsequent to the commencement of substantive discussions that
ultimately result in the Change of Control, the Company terminates the
Executive's employment with the Company without Cause, the Company shall:

          (1)  Pay to the Executive, within ten (10) business days following the
               Change of Control, a lump sum amount (net of any required
               withholding) equal to: (i) twelve (12) months of monthly base
               salary (at the highest monthly base salary rate in effect for the
               Executive in the twelve month period prior to the termination of
               his employment), plus (ii) the target bonus that could
<PAGE>

               have been payable to the Executive (assuming continued
               employment) during the year in which the Change of Control occurs
               based on bonus arrangements in effect at any time during the
               twelve month period immediately prior to the termination of his
               employment; and

          (2)  Provide the Executive and his dependents with life, accident,
               health and dental insurance substantially similar to that which
               the Executive was receiving immediately prior to the termination
               of his employment until the earlier of: (i) the date which is
               twelve (12) months following the Change of Control; or (ii) the
               date the Executive commences subsequent employment; and

          (3)  On the Change of Control, cause any unexercisable installments of
               any stock options of the Company or any subsidiary or affiliate
               of the Company held by the Executive on the Executive's last date
               of employment with the Company that have not expired to become
               exercisable on the Change of Control; provided, however, that:
                                                     --------  -------
               (i) such acceleration of exercisability shall not occur as to any
               option if the Change of Control does not occur within the period
               within which the Executive may exercise such option after a
               termination of employment in accordance with the provisions of
               the relevant option agreement and option plan; and (ii) any such
               acceleration of exercisability shall not extend the period after
               a termination of employment within which any option may be
               exercised by the Executive in accordance with the provisions of
               the relevant option agreement and option plan; and

          (4)  On the Change of Control, cause any unvested portion of any
               qualified or non-qualified capital accumulation benefits to
               become immediately vested (subject to applicable law);

provided, however, that any amounts and benefits set forth in this Section 2
--------  -------
shall be reduced by any and all other severance or other amounts or benefits
paid or payable to the Executive as a result of the termination of his or her
employment.

     3. Termination Following a Change of Control.
        -----------------------------------------

     (a) Notwithstanding the provisions of Section 1 and 2 above, if, at any
time during a period commencing with a Change of Control and ending eighteen
months after such Change of Control, the Company terminates the Executive's
employment without Cause or the Executive terminates his employment with the
Company for "Good Reason" (provided, however, that any such termination by the
                           --------  -------
Executive must occur promptly (and in any event within 90 days) after the
occurrence of the event or events constituting "Good Reason"), the Company
shall:

          (1)  Pay to the Executive, within ten (10) business days following the
               Executive's last date of employment, a lump sum amount (net of
               any

                                       -2-
<PAGE>

               required withholding) equal to: (i) twelve (12) months of monthly
               base salary (at the highest monthly base salary rate in effect
               for such Executive in the twelve (12) month period prior to the
               termination of his or her employment), plus (ii) the target bonus
               that could have been payable to such Executive (assuming
               continued employment) during the year in which the termination of
               employment occurs based on bonus arrangements in effect
               immediately prior to the termination of his or her employment
               (all payments under Sections 1, 2 and this Section 3(a) being
               referred to collectively, as the "Severance Payments"); and

          (2)  Provide the Executive and his or her dependents with life,
               accident, health and dental insurance substantially similar to
               that which the Executive was receiving immediately prior to the
               termination of his or her employment until the earlier of: (i)
               the date which is twelve (12) months following the termination of
               the Executive's employment; or (ii) the date the Executive
               commences subsequent employment; and

          (3)  Cause any unexercisable installments of any stock options of the
               Company or any subsidiary or affiliate of the Company held by the
               Executive on the Executive's last date of employment with the
               Company that have not expired to become exercisable on such last
               date of employment; provided, however, that: (i) such
                                   --------  -------
               acceleration of exercisability shall not occur as to any option
               if the Change of Control does not occur within the period within
               which the Executive may exercise such option after a termination
               of employment in accordance with the provisions of the relevant
               option agreement and option plan; and (ii) any such acceleration
               of exercisability shall not extend the period after a termination
               of employment within which any option may be exercised by the
               Executive in accordance with the provisions of the relevant
               option agreement and option plan; and

          (4)  Cause any unvested portion of any qualified and non-qualified
               capital accumulation benefits to become immediately vested,
               subject to applicable law;

provided, however, that any amounts and benefits set forth in this Section 3
--------  -------
shall be reduced by any and all other severance or other amounts or benefits
paid or payable to the Executive as a result of the termination of his or her
employment.

     (b) For purposes of Section 3 above, "Good Reason" shall mean the
occurrence of one or more of the following events following a Change of Control,
as the case may be: (i) the assignment to the Executive of any duties
inconsistent in any adverse, material respect with his position, authority,
duties or responsibilities immediately prior to the Change of Control or any
other action by the Company which results in a material diminution in such
position, authority, duties or responsibilities; (ii) a material reduction in
the aggregate of the Executive's base or incentive compensation or the
termination of the Executive's rights to any employee benefits

                                      -3-

<PAGE>

immediately prior to the Change of Control, except to the extent any such
benefit is replaced with a comparable benefit, or a reduction in scope or value
thereof; or (iii) a relocation of the Executive's place of business which
results in the one-way commuting distance for the Executive increasing by more
than 25 miles from the location thereof immediately prior to the Change of
Control (provided, however, that travel consistent with past practices for
         --------  -------
business purposes shall not be considered "commuting" for purposes of this
clause (iii)) or (iv) a failure by the Company to obtain the agreement
referenced in Section 4(f).

     4. General.
        -------

     (a) In the event the Executive's employment with the Company is terminated
by the Company for "Cause", or the Executive terminates his employment with the
Company other than during the specific time periods set forth in Section 3 or
for any reason other than Good Reason, the Executive shall not be entitled to
the severance benefits or other considerations described herein by virtue of
this Agreement.

     (b) For purposes of this Agreement, "Change of Control" shall mean the
closing of: (i) a merger, consolidation, liquidation or reorganization of the
Company into or with another Company or other legal person, after which merger,
consolidation, liquidation or reorganization the capital stock of the Company
outstanding prior to consummation of the transaction is not converted into or
exchanged for or does not represent more than 50% of the aggregate voting power
of the surviving or resulting entity; (ii) the direct or indirect acquisition by
any person (as the term "person" is used in Section 13(d)(3) or l4(d)(2) of the
Securities Exchange Act of 1934, as amended) of more than 50% of the voting
capital stock of the Company, in a single or series of related transactions; or
(iii) the sale, exchange, or transfer of all or substantially all of the
Company's assets (other than a sale, exchange or transfer to one or more
entities where the stockholders of the Company immediately before such sale,
exchange or transfer retain, directly or indirectly, at least a majority of the
beneficial interest in the voting stock of the entities to which the assets were
transferred).

     (c) For purposes of this Agreement, "Cause" shall mean: (i) the commission
by the Executive of a felony, either in connection with the performance of his
obligations to the Company or which adversely affects the Executive's ability to
perform such obligations; (ii) gross negligence, breach of fiduciary duty or
breach of any confidentiality, non-competition or developments agreement in
favor of the Company; or (iii) the commission by the Executive of an act of
fraud or embezzlement or other acts in intentional disregard of the Company
which result in loss, damage or injury to the Company, whether directly or
indirectly.

     (d) Notwithstanding anything to the contrary in this Agreement, if any
portion of any payments received by the Executive from the Company (whether
payable pursuant to the terms of this Agreement or any other plan, agreement or
arrangement with the Company, its successors or any person whose actions result
in a change of control of the Company) shall be subject to tax imposed by
Section 4999 of the Internal Revenue Code of 1986, as amended or any successor
statutory provision, the Company shall pay to the Executive such additional
amounts as are necessary so that, after taking into account any tax imposed by
Section 4999 (or any successor

                                       -4-
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statutory provision), and any federal and state income taxes payable on any such
tax, the Executive is in the same after-tax position that he or she would have
been if such Section 4999 (or any successor statutory provision) did not apply
and no payments were made pursuant to this Section 4(d). The Executive and the
Company shall each reasonably cooperate with the other in connection with any
administrative or judicial proceedings concerning the existence or amount of
liability for Excise Tax with respect to the Payments. All determinations
required to be made under this Section 4(d), including whether a Gross-Up
Payment is required and the amount of such Gross-Up Payment, shall be made by
the Company, after consultation with its tax and accounting advisors.

     (e) The parties hereto expressly agree that the payments by the Company to
the Executive in accordance with the terms of this Agreement will be liquidated
damages, and that the Executive shall not be required to mitigate the amount of
any payment provided for in this Agreement by seeking other employment or
otherwise, nor shall any profits, income, earnings or other benefits from any
source whatsoever create any mitigation, offset, reduction or any other
obligation on the part of the Executive.

     (f) Except as otherwise provided herein, this Agreement shall be binding
upon and inure to the benefit of the Company and any successor (whether direct
or indirect, by purchase, merger, consolidation, reorganization or otherwise) of
the Company; provided, however, that as a condition of closing any transaction
             --------  -------
which results in a Change of Control, the Company shall obtain the written
agreement of any successor (whether direct or indirect, by purchase, merger,
consolidation, reorganization or otherwise) of the Company to be bound by the
provisions of this Agreement as if such successor were the Company and for
purposes of this Agreement, any such successor of the Company shall be deemed to
be the "Company" for all purposes.

     (g) Nothing in this Agreement shall create any obligation on the part of
the Company or any other person to continue the employment of the Executive. If
the Executive elects to receive the severance and benefits set forth in Sections
1, 2 or 3, the Executive shall not be entitled to any other salary continuation
or severance benefits in the event of his cessation of employment with the
Company.

     (h) Nothing herein shall affect the Executive's obligations under any key
employee, non-competition, confidentiality, option or similar agreement between
the Company and the Executive currently in effect or which may be entered into
in the future.

     (i) This Agreement shall be governed by and construed in accordance with
the laws of the Commonwealth of Massachusetts. This Agreement constitutes the
entire Agreement between the Executive and the Company concerning the subject
matter hereof and supersedes any prior negotiations, understandings or
agreements concerning the subject matter hereof, whether oral or written, and
may be amended or rescinded only upon the written consent of the Company and the
Executive. The invalidity or unenforceability of any provision of this Agreement
shall not affect the other provisions of this Agreement and this Agreement shall
be construed and reformed to the fullest extent possible. The Executive may not
assign any of his rights or obligations under this Agreement; the rights and
obligations of the Company under this

                                      -5-
<PAGE>


Agreement shall inure to the benefit of, and shall be binding upon, the
successors and assigns of the Company. This Agreement may be executed in any
number of counterparts, all of which taken together shall constitute one and the
same instrument.

        IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
executed as of the date first written above.


                                        The Company:
                                        -----------

                                        PARAEXEL INTERNATIONAL CORPORATION

                                        By:  /s/ Joseph H. Von Rinckenbach
                                             ------------------------------

                                        Name:
                                             ------------------------------

                                        Title:
                                             ------------------------------

                                        The Executive:
                                        -------------

                                        Signature: /s/ James F. Winschel, Jr.
                                                   -------------------------

                                        Printed Name: JAMES F. WINSCHEL, JR.
                                                      ----------------------

                                      -6-
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                                                                       Exhibit A
                                                                       ---------
                                AGREEMENT/WAIVER
                                ----------------

     It is hereby agreed by and between __________________ (the "Executive") and
PAREXEL International Corporation (together with all subsidiaries and affiliates
hereinafter referred to as the "Company"), for good and sufficient consideration
more fully described below, that:

     1. Consideration. The Company will provide the Executive with the amounts
        -------------
and benefits described in Sections 1, 2 and 3 of the Change of Control/Severance
Agreement entered into by the Company and the Executive, dated
_________________, (the "Agreement"), subject to the terms and conditions of
such Agreement. The Executive understands that payment of and all such amounts
and benefits are conditioned upon the Executive signing this agreement.

     2. Settlement of Amounts Due the Executive. The Executive agrees that the
        ---------------------------------------
amounts set forth above in Section 1, together with any amounts previously
provided to the Executive by the Company, shall be complete and unconditional
payment, settlement, satisfaction and accord with respect to all obligations and
liabilities of the Company and any of its affiliated companies (including their
respective successors, assigns, shareholders, officers, directors, employees
and/or agents) to the Executive, and all claims, causes of action and damages by
the Executive against the Company and/or any such other parties regarding the
Executive's employment with and termination from employment with the Company,
including, without limitation, all claims for back wages, salary, draws,
commissions, bonuses, vacation pay, equity compensation, expenses, compensation,
severance pay, attorney's fees, compensatory damages, exemplary damages, or
other costs or sums.

     3. Release.
        -------

     (a) In exchange for the amounts and benefits described in Section 1 above
and other good and valuable consideration, receipt of which is hereby
acknowledged, the Executive and his representatives, agents, estate, successors
and assigns, absolutely and unconditionally hereby release and forever discharge
the Company, its affiliated companies and/or their successors, assigns,
directors, shareholders, officers, employees and/or agents, both individually
and in their official capacities, (the "Releasees"), from any and all actions or
causes of action, suits, claims, complaints, contracts, liabilities, agreements,
promises, debts and damages, controversies, judgments, rights and demands,
whether existing or contingent, known or unknown, which arise under the
Agreement. This release is intended by the Executive to be all encompassing and
to act as a full and total release of any claims that the Executive may have or
has had against the Releasees under the Agreement, including, but not limited
to, any federal, state or local law or regulation dealing with either employment
or employment discrimination such as those laws or regulations concerning
discrimination on the basis of age, race, color, religion, creed, sex,
sexualorientation, national origin, ancestry, marital status, physical or mental
disability, any veteran status or any military service or application for any
military service; any contract, whether oral or written, express or implied; or
common law.
<PAGE>

     (b) The Executive agrees not only to release and discharge the Releasees
from any and all claims as stated above that the Executive could make on his/her
own behalf or on behalf of others, but also those claims which might be made by
any other person or organization on behalf of the Executive, and the Executive
specifically waives any right to become, and promises not to become, a member of
any class in a case in which a claim or claims against the Releases are made
involving any matters which arise out of, or in connection with, the Agreement.
Nothing in this agreement is to be construed as an admission by the Releasees of
any liability or unlawful conduct whatsoever.

     4. Waiver of Rights and Claims Under the Age Discrimination and Employment
        -----------------------------------------------------------------------
Act of 1967.
-----------

     (a) The Executive has been informed that since he is 40 years of age or
older, he has or might have specific rights and/or claims under the Age
Discrimination and Employment Act of 1967. In consideration for the amounts
described in Section 1 hereof, the Executive specifically waives such rights
and/or claims to the extent that such rights and/or claims arose prior to the
date this Agreement was executed.

     (b) The Executive was advised by the Company of his right to consult with
an attorney prior to executing this Agreement.

     (c) The Executive was further advised when he was presented by the Company
with the original draft of this Agreement on _______, 200, that he had at least
21 days within which to consider its terms and to consult with or seek advice
from an attorney or any other person of his/her choosing, until the close of
business on ___________, 200_.

     5. Confidentiality. The Executive agrees he shall not divulge or publish,
        ---------------
directly or indirectly, any information whatsoever regarding the substance,
terms or existence of the Agreement or this agreement and/or any discussions or
negotiations relating to the Agreement or this agreement to any person or
organization, except to his immediate family members, counsel or accountant, and
unless required under law or court order.

     6. Representations and Governing Law.
        ---------------------------------

     (a) This agreement represents the complete and sole understanding between
the parties regarding the subject matter hereto. This agreement may not be
modified, altered or rescinded except upon written consent of the Company and
Executive. The invalidity or unenforceability of any provision of this agreement
shall not affect the other provisions of this agreement, but this agreement
shall be revised, construed and reformed to the fullest extent possible to
effectuate the purposes of this agreement. This agreement shall be binding upon
and inure to the benefit of the Company and the Executive and their respective
heirs, successors and assigns. The parties agree that the Company will not have
an adequate remedy if the Executive fails to comply with Sections 3, 4, and 5
hereof and that damages will not be readily ascertainable, and that in the event
of such failure, the Executive shall not oppose any application by the Company
requiring a

                                      A-2
<PAGE>

decree of specific performance or an injunction enjoining a breach of this
agreement. If the Executive breaches any of his/her obligations hereunder, he
shall forfeit all right to payments pursuant to Section 1.

     (b) This agreement shall be governed by and construed in accordance with
the laws of the Commonwealth of Massachusetts, without giving effect to the
principles of conflicts of law thereof.

     (c) The Executive represents that he has read this agreement, fully
understands the terms and conditions of such agreement, and is voluntarily
executing the same. In entering into this agreement, the Executive does not rely
on any representation, promise or inducement made by the Releases, with the
exception of the consideration described in this document.

     7. Effective Date. The Executive may revoke this agreement during the
        --------------
period of seven (7) days following its execution by the Executive, and this
agreement shall not become effective or enforceable until this revocation period
has expired.


                                     The Company:
                                     -----------

                                     PAREXEL INTERNATIONAL CORPORATION

                                     By:
                                        -------------------------------

                                     Name:
                                          -----------------------------

                                     Title:
                                           ----------------------------

                                     Date:
                                          -----------------------------

                                     The Executive:
                                     -------------

                                     Signature:
                                               ------------------------

                                     Printed Name:
                                                  ---------------------

                                     Date:
                                          -----------------------------

                                      A-3