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                                                                   EXHIBIT 10.16


                            THIRD AMENDMENT OF LEASE


     AMENDMENT OF LEASE, dated as of October 15, 1998, by and between the
NATIONAL FIRE PROTECTION ASSOCIATION (hereinafter, "Landlord"), a Massachusetts
not-for-profit corporation, and ATLANTIC DATA SERVICES, INC., a Massachusetts
corporation (hereinafter, "Tenant").

A.   Landlord and Tenant are parties to a Lease dated as of April 1, 1995
     (hereinafter, the "Lease"), a First Amendment of Lease dated July 31, 1995,
     and a Second Amendment of Lease dated January 15, 1997 covering certain
     premises located on the third floor of the National Fire Protection
     Headquarters Building (hereinafter, the "Building"), located at
     Batterymarch Park, Quincy, MA.

B.   Landlord and Tenant desire to further amend the Lease to provide for the
     addition to the premises demised thereunder of one thousand five hundred
     forty (1,540) rentable square feet located on the third floor of the
     Building (hereinafter called the "Additional Space").

     NOW THEREFORE, in consideration of Ten Dollars ($10.00) and other good and
valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the Landlord and Tenant hereby agree as follows:

     1.   The following definition contained in ss.1.2 of the Lease as Amended
          by the First Amendments, are hereby amended as follows:

          a.   BASIC RENT: no changes

          b.   PREMISES RENTABLE AREA: Delete from the definition of Premises
               Rentable Area the number 23,742 and insert in lieu thereof the
               number 25,282.

          c.   ESCALATION FACTOR: Delete from the definition of Escalation
               Factor the percentage 16.78 and insert in lieu thereof the
               percentage 17.86.

     2.   Exhibit F to the Lease is hereby amended by the addition to the space
          therein described of the space shown as ADS THIRD AMENDMENT EXPANSION
          SPACE, on AMENDMENT EXHIBIT 1, attached hereto.

     3.   The tenant agrees to accept the said premises "as is" in its current
          condition. Should the tenant desire to make any improvements to the
          premises, Tenant shall be responsible for all such improvements
          including obtaining all permits. Tenant shall indemnify and hold
          harmless the landlord from any loss, cost, injury or damage as a
          result of any improvements to the premises. In all additional ways,
          the improvements shall comply with the provisions of Section 5.2 of
          the Lease. Tenant shall, as part of the approval required under said
          Section 5.2, submit plans for all improvements to be undertaken.

     4.   The provisions of this Third Amendment to Lease shall be effective as
          of November 1, 1998.

     5.   The Lease, as hereby modified, shall continue in full force and effect
          and is hereby confirmed, approved and ratified.


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     IN WITNESS WHEREOF, the parties hereto have caused this Third Amendment of
Lease to be executed as a sealed instrument as of the date first written above.


LANDLORD                                     TENANT

NATIONAL FIRE PROTECTION
ASSOCIATION



BY: /s/ National Fire Protection Agency      BY: /s/ Paul K. McGrath
    -------------------------------------        ------------------------------
                                                 Senior Vice President & CFO