<PAGE>

                                                                   EXHIBIT 4.3


THE SECURITIES REPRESENTED HEREBY HAVE BEEN ACQUIRED IN A TRANSACTION NOT
INVOLVING ANY PUBLIC OFFERING AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933 OR THE SECURITIES LAWS OF ANY STATE. SUCH SECURITIES MAY NOT BE SOLD
OR TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN EXEMPTION THEREFROM
UNDER SAID ACT AND LAWS.


                      AMERICAN PSYCH SYSTEMS HOLDINGS, INC.

                     -----------------------------------
                             STOCK PURCHASE WARRANT
                     -----------------------------------

                                 August 18, 2000

     1. GRANT. AMERICAN PSYCH SYSTEMS HOLDINGS, INC., a Delaware corporation
(hereinafter the "Company"), for value received hereby grants to KENNETH A.
KESSLER, M.D., a resident of Maryland (collectively with successors and
registered assigns, "Holder") under the terms herein the right to purchase
Seventy-Five Thousand (75,000) of the fully paid and nonassessable shares of the
Company's authorized but unissued $0.001 par value Class A common stock at a
purchase price of One Cent ($0.01) per share, as such number of shares and price
may be adjusted in accordance herewith.

     2. NOTE. This Warrant has been issued under the terms of a Promissory Note
of the Company dated this date (the "Note"). The Holder is entitled to the
benefits of the Note and all of the exhibits thereto, and reference is made
thereto for a description of all rights and remedies thereunder.

     3. DEFINITIONS.

        (a) COMMON STOCK is defined as the $0.001 par value Class A Common
Shares of the Company.

        (b) WARRANT SHARES is defined as the shares of Common Stock issued or
issuable hereunder, referred to collectively.

     4. TERM. The right to exercise this Warrant shall expire three (3) years
after the date hereof.

     5. EXERCISE PRICE. Unless otherwise provided herein, the exercise price for
each Warrant Share purchasable hereunder shall be One Cent ($0.01).

     6. EXCHANGE OF SHARES FOR EXERCISE PRICE. The Holder at its option may
provide the Exercise Price under this Warrant by reducing the number of shares
for which the Warrant is otherwise exercisable by the number of shares having
fair market value equal to the Exercise Price. In the case where the Exercise
Price is less than the value of one (1) share, the said Exercise Price will
increase to equal the fair market value of one (1) share.

     7. HOLDER'S REDEMPTION RIGHTS. The Holder will share ratably in any
redemption of stock by the Company. If the Company shall redeem or otherwise
purchase for value any of its Common Stock prior to full exercise of this
Warrant, the Holder, at its option, may receive, at


<PAGE>

the time of such redemption or purchase, the same proceeds it would have been
entitled to receive if this Warrant had been exercised in full prior to such
redemption or purchase.

     8. EXERCISE PROCEDURE. This Warrant may be exercised by presenting it and
tendering the exercise price in legal tender or by bank's, cashier's or
certified check to the Company at its address, along with a written subscription
substantially in the form of EXHIBIT A hereof. The date on which this Warrant is
thus surrendered, accompanied by tender or payment as hereinbefore or
hereinafter provided, is referred to herein as the "Exercise Date". The Company
shall forthwith at its expense (including the payment of issue taxes) issue and
deliver the proper number of shares, and such shares shall be deemed issued for
all purposes as of the opening of business on the Exercise Date notwithstanding
any delay in the actual issuance.

     9. SALE OR EXCHANGE OF COMPANY OR ASSETS. If prior to issuance of stock
under this Warrant the Company sells or exchanges all or substantially all of
its assets, or all or substantially all the Common Stock of the Company is sold
or exchanged to any party other than the Holder, then the Holder at its option
may receive, in lieu of the stock otherwise issuable hereunder, such money or
property it would have been entitle to receive if this Warrant had been
exercised prior to such sale or exchange.

     10. SALE OF WARRANT OR SHARES. Neither this Warrant nor other shares of
common stock issuable upon exercise hereof have been registered under the
Securities Act of 1933, as amended, or under the securities laws of any state.
Neither this Warrant nor any shares issued pursuant to it may be sold,
transferred, pledged or hypothecated in the absence of (i) an effective
registration statement for this Warrant or the subject shares, as the case may
be, under the Securities Act of 1933, as amended, and such registration or
qualification as may be necessary under the securities laws of any state, or
(ii) an opinion of counsel reasonably satisfactory to the Company that such
registration or qualification is not required. The Company shall cause any
certificate or certificates evidencing all or any of the shares issued upon
exercise hereof prior to said registration and qualification to bear the
following legend:

          THE SHARES EVIDENCED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
          UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR UNDER THE SECURITIES
          LAWS OF ANY STATE. THE SHARES MAY NOT BE SOLD, TRANSFERRED, PLEDGED OR
          HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT
          UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND SUCH REGISTRATION OR
          QUALIFICATION AS MAY BE NECESSARY UNDER THE SECURITIES LAWS OF ANY
          STATE, OR AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT SUCH
          REGISTRATION OR QUALIFICATION IS NOT REQUIRED.

     11. CERTAIN STOCK ACTIONS. If the Company shall subdivide its shares of
Common Stock by stock split, stock dividend or otherwise, the number of Warrant
Shares issuable upon the exercise hereof shall proportionately increase and the
Exercise Price shall proportionately decrease and, conversely, if the Company
shall combine its outstanding shares of Common Stock by stock combination,
reverse split or otherwise, the number of Warrant Shares issuable upon exercise
hereof shall proportionately decrease and the Exercise Price shall
proportionately increase.

     12. REORGANIZATION AND MERGERS. In case of any capital reorganization or
any reclassification of the capital stock of the Company or in case of the
consolidation or merger of the Company with another corporation (other than a
consolidation or merger in which the

                                       2


<PAGE>

ompany is the surviving corporation), or in case of any sale, transfer or other
disposition to another corporation of all or substantially all of the property,
assets, business and goodwill of the Company, the Holder of this Warrant shall
thereafter be entitled to purchase (and it shall be a condition to the
consummation of any such reorganization, reclassification, consolidation,
merger, sale, transfer or other disposition that appropriate provision shall be
made so that such Holder shall thereafter be entitled to purchase) the kind and
amount of shares of stock and other securities and property receivable in such
transaction which the Warrant entitled the Holder to purchase immediately prior
to such capital reorganization, reclassification of capital stock,
consolidation, merger, sale, transfer or other disposition, and in any such case
appropriate adjustments shall be made in the application of the provisions of
this Section 12 with respect to rights and interest thereafter of the Holder of
this Warrant to the end that the provisions of this Section 12 shall thereafter
be applicable, as near as reasonably may be, in relation to any shares or other
property thereafter purchasable upon the exercise of this Warrant.

     13. FRACTIONAL SHARES. No certificate for fractional shares shall be issued
upon the exercise of this Warrant, but in lieu thereof the Company shall
purchase any such fractional interest calculated to the nearest cent.

     14. NOTICE. Whenever the Exercise Price is adjusted as herein provided, the
Company shall forthwith deliver to the Holder a statement signed by the
President or the Treasurer of the Company stating the adjusted Exercise Price
and number of shares determined as herein specified. Such statement shall show
in detail the facts requiring such adjustment, including a statement of the
consideration received by the Company for any additional stock issued.

     15. LIQUIDATION OR DISSOLUTION. In case the Company shall, at any time
during the term of this Warrant, dissolve, liquidate or wind up its affairs, the
Holder shall be entitled, upon the exercise of this Warrant and prior to any
such distribution in dissolution or liquidation, to receive on such exercise, in
lieu of the shares of Common Stock which the Holder would have been entitled to
receive, the same kind and amount of assets as would have been distributed or
paid to the Holder upon any such dissolution, liquidation or winding up with
respect to such shares of Common Stock had the Holder been the holder of record
of such shares of Common Stock on the record date for the determination of those
holders of Common Stock entitled to receive any such liquidation distribution.

     16. TRANSFER. This Warrant shall be registered on the books of the Company
which shall be kept at its principal office for that purpose. This Warrant and
all rights hereunder are not transferable to any Person, in whole or in part,
except a member of the immediate family of the Holder or a trust for their
benefit.

     17. REPLACEMENT OF WARRANT. At the request of the Holder and on production
of evidence reasonably satisfactory to the Company of the loss, theft,
destruction or mutilation of this Warrant and (in the case of loss, theft, or
destruction) if required by the Company, upon delivery of an indemnity agreement
with surety in such reasonable amount as the Company may determine thereof, the
Company at its expense will issue in lieu thereof a new Warrant of like tenor.

     18. INVESTMENT COVENANT. The Holder by its acceptance hereof covenants that
this Warrant is, and any stock issued hereunder will be, acquired for investment
purposes, and that the Holder will not distribute the same in violation of any
state or federal law or regulation.

                                       3

<PAGE>

     IN WITNESS WHEREOF, the Company has caused this Warrant to be signed on its
behalf by its undersigned officer, and its corporate seal to be hereunto
affixed, as of the date first above written.

SEAL:                                    AMERICAN PSYCH SYSTEMS
                                         HOLDINGS, INC.



Attest: /s/ Laura Fisher                 By: /s/ Kenneth A. Kessler
       ----------------------------          -------------------------------
                                             Name: Kenneth A. Kessler, MD
            Laura Fisher, Secretary          Title: President & CEO
       ----------------



STATE OF   Maryland           )
         ---------------------
                              )  ss:
COUNTY OF  Montgomery         )
          --------------------

         Personally appeared before me, a Notary Public in and for the
jurisdiction aforesaid, Kenneth A. Kessler, MD, made known to me as the
President & CEO of American Psych Systems Holdings, Inc., a Delaware
corporation, who acknowledged the foregoing as the true act and deed of such
corporation this 21st day of August, 2000.

                                       /s/ Joseph Magha
                                       -------------------------------------
                                       Notary Public

[Notarial Seal]

My Commission Expires: September 25, 2001
                       --------------------


<PAGE>

                                    EXHIBIT A

                            IRREVOCABLE SUBSCRIPTION

To:  American Psych Systems Holdings, Inc.

Ladies and Gentlemen:

The undersigned hereby elects to exercise its right under the attached Warrant
by purchasing ______________________ shares of the Class A common stock of the
Company, and hereby irrevocably subscribes to such issue. The certificates for
such shares shall be issued in the name of


-------------------------------------------------------------------------------
(Name)

-------------------------------------------------------------------------------
(Address)

-------------------------------------------------------------------------------
(Taxpayer Number)

and deliver to:

-------------------------------------------------------------------------------
(Name)

-------------------------------------------------------------------------------
(Address)

The exercise price of $_____________ is enclosed [OR _____________ SHARES UNDER
THE WARRANT ARE FORFEITED PURSUANT TO SECTION 6 OF THE WARRANT].

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

Signed:                                          (Name of Holder, Please Print)
       -----------------------------------------

-------------------------------------------------------------------------------
(Address)

-------------------------------------------------------------------------------
(Signature)

                                      A-1