<PAGE>

                                                                   EXHIBIT 10.67

                             TERMINATION AGREEMENT
                             ---------------------

     This TERMINATION AGREEMENT dated as of March 29, 2001, is among NMT
Medical, Inc., a Delaware corporation, NMT Heart, Inc., a Delaware corporation,
NMT Investments Corp., a Massachusetts corporation, NMT NeuroSciences
(International), Inc., a Delaware corporation, NMT NeuroSciences (US), Inc., a
Delaware corporation, NMT NeuroSciences (IP), Inc., a Delaware corporation, and
NMT Neurosciences Innovasive Systems, Inc., a Florida corporation (collectively,
the "Borrowers"), and Brown Brothers Harriman & Co. (the "Lender").  Capitalized
terms used herein and not otherwise defined shall have the meanings set forth in
the Credit Agreement (as defined below).

                                   Recitals
                                   --------

     A.  Pursuant to the terms of that certain Credit Agreement dated as of
September 13, 1999, among the Borrowers and the Lender (as amended to date, the
"Credit Agreement"), the Lender has provided certain financial accommodations to
the Borrowers;

     B.  The Borrowers have (i) repaid to the Lender all outstanding Loans under
the Credit Agreement and paid all of their other obligations thereunder
(including, without limitation, all outstanding fees, charges and disbursements
of counsel for the Lender) and (ii) delivered to the Lender any original Letters
of Credit outstanding under the Credit Agreement marked "terminated"; and

     C.  The Borrowers and the Lender desire to terminate the Credit Agreement
and the ability of the Borrowers to borrow thereunder.

     NOW, THEREFORE, in consideration of the premises and other good and
valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the parties hereby agree as follows:

     Section 1.  Termination of Loan Documents.  The Credit Agreement and each
of the security documents and pledge agreements referred to in Section 4.01 of
the Credit Agreement or otherwise provided to the Lender in connection with the
Credit Agreement and the transactions contemplated thereby (collectively, with
the Credit Agreement, the "Loan Documents") are hereby terminated and shall be
of no further force or effect, except, as to each such Loan Document, for
provisions therein that expressly survive the termination of such Loan Document,
including, without limitation, Sections 2.11, 2.12 and 8.03 of the Credit
Agreement in accordance with Section 8.05 thereof.  The Borrowers hereby
terminate all obligations of the Lender to make any further extensions of credit
pursuant to the Credit Agreement.  At the request of the Borrowers, the  Lender
shall this day deliver to the Borrowers the promissory note executed by the
Borrowers in favor of the Lender pursuant to the Credit Agreement.

     Section 2.  Release of Collateral.  The Lender hereby terminates and
releases any security interests in or liens on any property of the Borrowers or
any other party heretofore granted to secure the payment or performance of the
obligations of the Borrowers under the Loan Documents (collectively, the
"Collateral"), and hereby releases and reassigns all such Collateral
<PAGE>

to the Borrowers or such other parties (as applicable). At the request of the
Borrowers, the Lender shall this day return to the Borrowers all stock
certificates and other Collateral delivered to the Lender.

     Section 3.  Further Assurances.  The parties to this Agreement shall, upon
the reasonable request of the other party and at the reasonable expense of the
Borrowers (including, without limitation, the payment of the reasonable fees,
charges and disbursements of counsel for the Lender), take any and all actions
and execute any and all documents (including, but not limited to, the UCC-3
termination statements attached hereto as Exhibit A and the patent assignment
termination attached hereto as Exhibit B) necessary or desirable to effectuate
the terms and intent of this Agreement, including, without limitation, the
termination of the Loan Documents as provided herein and the release of the
Collateral. Without limiting the generality of the foregoing, at the request of
the Borrowers, the Lender hereby reconfirms its authorization (as set forth in
that certain Release of Security Interest Agreement dated as of March 20, 2000)
of J.H. Whitney & Co. and Whitney Subordinated Debt Fund, L.P. (collectively,
"Whitney") to, at the expense of the Borrowers, execute and deliver to the
Borrowers all patent security interest terminations or other documentation, and
take any and all actions, deemed necessary or advisable by the Borrowers to
evidence, implement or confirm the termination of Whitney's security interest in
or lien on the non-United States patents of the Borrowers and/or the affiliates
of the Borrowers (such security interest in and/or lien on such non-United
States patents to have been held by Whitney for the benefit of the Lender
pursuant to that certain Amendment No. 2 to Subordinated Note and Common Stock
Purchase Agreement, dated as of September 13, 1999, by and among NMT Medical,
Inc. and Whitney).

     Section 4.  Miscellaneous.  This Agreement (i) may be executed in any
number of counterparts, which together shall constitute one instrument, (ii)
shall be governed by and construed in accordance with the laws (other than the
conflict of laws rules) of the Commonwealth of Massachusetts and (iii) shall
bind and inure to the benefit of the parties hereto and their respective
successors and assigns.

                                 [END OF TEXT]

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     Each of the undersigned has caused this Agreement to be executed and
delivered by its duly authorized officer as of the date hereof.


                                    NMT MEDICAL, INC.


                                    By: /s/ John E. Ahern
                                       ------------------------------
                                       Name: John E. Ahern
                                       Title Chief Executive Officer


                                    NMT HEART, INC.


                                    By: /s/ John E. Ahern
                                       ------------------------------
                                       Name: John E. Ahern
                                       Title President


                                    NMT INVESTMENTS CORP.


                                    By: /s/ John E. Ahern
                                       ------------------------------
                                       Name: John E. Ahern
                                       Title President


                                    NMT NEUROSCIENCES
                                    (INTERNATIONAL), INC.


                                    By: /s/ John E. Ahern
                                       ------------------------------
                                       Name: John E. Ahern
                                       Title President


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<PAGE>

                                    NMT NEUROSCIENCES (US), INC.


                                    By: /s/ John E. Ahern
                                       ------------------------------
                                       Name: John E. Ahern
                                       Title President


                                    NMT NEUROSCIENCES (IP), INC.


                                    By: /s/ John E. Ahern
                                       ------------------------------
                                       Name: John E. Ahern
                                       Title President


                                    NMT NEUROSCIENCES
                                    INNOVASIVE SYSTEMS, INC.


                                    By: /s/ John E. Ahern
                                       ------------------------------
                                       Name: John E. Ahern
                                       Title President


                                    BROWN BROTHERS HARRIMAN & CO.


                                    By: /s/ Gregory S. Pachus
                                       ------------------------------
                                       Name: Gregory S. Pachus
                                       Title Vice President

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