Exhibit 10.1

Execution Copy

December 20, 2007

Mr. Robert Essner

Wyeth

Five Giralda Farms

Madison, NJ 07940

Dear Bob:

This letter agreement is to confirm our mutual understanding and agreement
regarding the succession arrangements to be made in connection with your
announced retirement from Wyeth (the “Company”) and your resignation as Chief
Executive Officer of the Company effective as of close of business on
December 31, 2007. For these purposes, reference is made to your employment
agreement with the Company dated as of January 25, 2007 (your “Employment
Agreement”). Capitalized terms, not otherwise defined in this letter agreement,
shall continue to have the meaning set forth in the Employment Agreement. Should
your employment terminate for any reason prior to the close of business on
December 31, 2007, this letter agreement (other than Appendix A) will be of no
force and effect and the Employment Agreement without regard to this letter
agreement (other than Appendix A) will control.

Effective as of the date you sign this letter agreement where indicated below,
your Employment Agreement will be deemed amended to reflect the provisions of
this letter agreement. To that end, first, in connection with your announced
retirement and the Company’s succession planning, you hereby irrevocably agree
that none of the announcement of your successor as Chief Executive Officer, any
actions taken by the Company following such announcement in furtherance of the
succession planning, nor any of the provisions of this letter agreement, shall
at any time hereafter constitute a basis for your claiming “Good Reason” for any
purpose under your Employment Agreement.

Second, you and the Company hereby agree that:

 

  (a) Effective as of the close of business on December 31, 2007, you shall
resign from your position as Chief Executive Officer of the Company and from all
boards of directors of any subsidiaries of the Company.

 

  (b) Effective as of the close of business on December 31, 2007, you will
continue to be an employee of the Company and to serve as the Chairman of the
Board until December 31, 2008 (or until such earlier date as your employment may
terminate, for any reason) (such period of employment, the “Transition Period”).

 

  (c) During the Transition Period, your duties and responsibilities will be
those customarily performed by the Chairman of the Board and as are reasonably
requested by the Board to aid in the transition of your named successor as Chief
Executive Officer.

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  (d) During the Transition Period: (1) your Base Salary shall remain $1,728,500
for all purposes under your Employment Agreement; (2) you shall continue to be
entitled to earn an annual bonus in respect of 2007 consistent with your
position within the Company during such year, to be paid in 2008 at the same
time as 2007 annual bonuses are paid to other senior executives (the “2007
Bonus”); and (3) you shall be entitled to earn an annual bonus in respect of
2008 (the “2008 Bonus”) consistent with your position within the Company during
such year, provided that such bonus will be no less than the greater of (x) the
2007 Bonus and (y) the annual bonus paid to you in 2007 in respect of 2006. The
2008 Bonus shall be paid in 2009 when 2008 annual bonuses are otherwise paid to
other senior executives and shall be subject to pro-ration on account of a
termination of employment prior to December 31, 2008, as described in paragraph
(f) below. Your equity incentive award for 2008 shall be determined and will be
made by the Compensation Committee at its meeting in April, 2008 at which such
awards are made to senior executives.

 

  (e) During the Transition Period, only thirty (30) days’ advance written
notice (unless otherwise waived by the other party) will be required to be given
by either you or the Company to terminate your employment for any reason (other
than on account of Cause, your death or the expiration of the Transition Period
on December 31, 2008, in each such case, no notice will be required).

 

  (f) If your employment terminates during the Transition Period for any reason
(other than for Cause) or upon the expiration of the Transition Period on
December 31, 2008, you shall be entitled to receive the payments and benefits
provided for in Section 5(f)(i) and 5(f)(ii) of your Employment Agreement,
except that, in lieu of the payment of a pro-rated bonus as provided for in
Section 5(f)(ii)(B) of your Employment Agreement, you shall instead be entitled
to a payment of a portion of the 2008 Bonus, prorated by a fraction, the
numerator of which is the number of calendar days that you were employed in 2008
and the denominator of which is 365 (the “Prorated 2008 Bonus Amount”), payable
in 2009 when 2008 annual bonuses are otherwise paid to the Company’s senior-most
executives. For the avoidance of doubt, this means that, effective on the close
of business on December 31, 2007, you shall not be entitled to receive any of
the payments or benefits provided in Section 5(f)(iii) of your Employment
Agreement. In addition, if the Company terminates your employment after
December 31, 2007 without Cause upon fewer than thirty days’ advance written
notice, you shall receive a lump sum payment equal to the Base Salary and the
Prorated 2008 Bonus Amount you would have received had you remained employed
with the Company through the expiration of such thirty-day notice period.

 

  (g) The Severance Agreement shall only apply and supersede the Employment
Agreement, as provided in Section 13(b) of the Employment Agreement, if a Change
in Control occurs while you are still employed with the Company.

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Your Employment Agreement is also hereby amended effective as of the date you
sign this letter agreement to reflect the changes set forth in Appendix A of
this letter agreement (which changes are necessary to comply with Section 409A
of the Code).

For the avoidance of doubt, the parties hereby acknowledge and agree that you
will continue to be bound by the provisions of Section 6 (Non-Competition),
Section 7 (Litigation Assistance), Section 8 (Non-Solicitation,
Confidentiality), Section 9 (Return of Documents and Company Property), and
Section 10 (Enforcement of Covenants), in each case of the Employment Agreement,
in accordance with their terms as currently in effect. The provisions of
Section 11 (Assumption of Agreement), Section 12 (Indemnification) and
Section 14 (Miscellaneous) of your Employment Agreement are incorporated by
reference into this letter agreement and made a part hereof.

The Employment Agreement (subject to the terms of Section 13(b) of the
Employment Agreement), as amended by this letter agreement, constitutes the
entire agreement among you and the Company with respect to the subject matter
contained therein and herein and supersedes all prior and contemporaneous
agreements, discussions, understandings and negotiations, whether written or
oral, with respect thereto.

The Company agrees that it will consult with you as to the content of any
proposed press release related to this letter agreement or the actions
contemplated hereby.

Please acknowledge your agreement to the foregoing amendments to your Employment
Agreement, and to the other provisions of this letter agreement, by signing
where indicated below and returning a copy of such executed letter agreement to
me.

Bob, on behalf of the entire Board, let me extend our great appreciation and
respect for you and all of your contributions to Wyeth.

 

Sincerely, /s/ Ivan Seidenberg Ivan Seidenberg Chairman of the Compensation and
Benefits Committee of the Board of Directors of Wyeth

 

Accepted and agreed this 20th day of December, 2007. /s/ Robert Essner Robert
Essner

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Appendix A

Section 409A Compliance

 

(a) For purposes of Section 5(f)(ii)(C) of your Employment Agreement, the timing
of any payment of performance-based equity awards to which you are entitled
shall only be made at the time such payment is provided for under the terms of
the applicable Equity Plans or agreements and shall not be accelerated in a
manner which would be impermissible under Section 409A of the Code.

 

(b) Payment due under Section 5(f)(iii)(A) of your Employment Agreement shall be
paid on the sixty-fifth day following your Separation from Service Date (as
defined below).

 

(c) Notwithstanding anything else contained in your Employment Agreement or this
letter agreement, if, as of your Separation from Service Date, you are a
Specified Employee (as defined below) and entitled to receive payments and/or
benefits under Section 5 of your Employment Agreement, then, except to the
extent that the Employment Agreement (as amended by this letter agreement as
applicable) does not provide for a “deferral of compensation” within the meaning
of Section 409A of the Code, the following shall apply:

(I) No payments shall be made and, except for the benefits described in
Section 5(f)(ii)(E) of your Employment Agreement (subject to this paragraph
(c)), no benefits shall be provided to you, in each case, during the six-month
period beginning on the Separation from Service Date or, if earlier, the date of
your death (such six-month period, the “Delay Period”).

(II) With respect to the benefits described in Section 5(f)(ii)(E) of your
Employment Agreement, during the Delay Period, you will reimburse the Company,
on the last business day of each such month, for the amount of any income
imputed to you under applicable tax rules as a result of any benefits provided
to you during all or any portion of such month. However, on the first business
day of the first month following the month in which the Delay Period ends, the
Company will make a one-time, lump-sum cash payment to you in an amount equal to
the sum of (x) the amounts otherwise payable to you under the Employment
Agreement during the Delay Period, (y) the reimbursement payments made by you in
accordance with this clause (II) above and (z) the amount of interest on the
foregoing at the applicable federal rate for instruments of less than one year.

For purposes of the foregoing, “Separation from Service Date” shall mean the
date of your “separation from service” within the meaning of
Section 409A(a)(2)(i)(A) of the Code and determined in accordance with the
default rules under Section 409A of the Code. “Specified Employee” shall mean a
“specified employee” within the meaning of Section 409A(a)(2)(B)(1) of the Code,
as determined in accordance with the uniform methodology and procedures adopted
by the Company and then in effect.

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(d) Section 14(d) of your Employment Agreement is hereby amended by adding to
the end of such section the following:

“Anything in this Agreement to the contrary notwithstanding, any reimbursement
payable to the Executive pursuant to any provisions of this Agreement or
pursuant to any plan or arrangement of the Company covered by this Agreement
shall be paid in accordance with the applicable terms thereof, except that any
such reimbursement that constitutes deferred compensation within the meaning of
Section 409A of the Code shall be paid no later than the last day of the
calendar year following the calendar year in which the related expense was
incurred. No amount reimbursed during any calendar year shall affect the amounts
eligible for reimbursement in any other calendar year. This Agreement is
intended to satisfy the requirements of Section 409A of the Code with respect to
amounts subject thereto and shall be interpreted and construed and shall be
performed by the parties consistent with such intent.”