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                                                                     EXHIBIT 4.1


                                                                  EXECUTION COPY


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                              IMPSAT CORPORATION,
                                   as Issuer


                                  IMPSAT S.A.,
                                  as Guarantor


                                      and


                             THE BANK OF NEW YORK,
                                   as Trustee



                              --------------------

                 Senior Guaranteed Notes Supplemental Indenture

                            Dated as of May 13, 1997

                              --------------------


                   12 1/8 % Senior Guaranteed Notes due 2003


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         FIRST SUPPLEMENTAL INDENTURE (this "Supplemental Indenture"), dated as
of May 13, 1997, among IMPSAT Corporation, a Delaware Corporation, as issuer
(the "Company"), IMPSAT S.A., an Argentine corporation, as guarantor (the
"Guarantor"), and THE BANK OF NEW YORK, as Trustee (the "Trustee").

                              W I T N E S S E T H:

         WHEREAS, in accordance with Section 9.01(e) of the Indenture relating
to the 12 1/8 % Senior Guaranteed Notes due 2003 of the Company, dated as of
July 30, 1996, among the Company, the Guarantor and the Trustee (the
"Indenture"), the Company, the Guarantor and the Trustee desire to amend certain
terms related to the definition of Permitted Investment, the Limitation on
Indebtedness covenant, the Limitation on Restricted Payments covenant and other
matters as provided for below;

         WHEREAS, all things necessary to make this Supplemental Indenture a
valid supplement to the Indenture according to the terms of this Supplemental
Indenture and the terms of the Indenture have been done;

         NOW THEREFORE, THE PARTIES HERETO AGREE AS FOLLOWS:

         SECTION 1.1 Certain Terms Defined in the Indenture. All capitalized
terms used herein without definition herein shall have the meanings ascribed
thereto in the Indenture.                              

         SECTION 1.2 Amendment of Section 1.01. (a) Section 1.01 of the
Indenture is hereby amended by adding the following definition:
                                             
         "Intermediary Documents" means documents, substantially in the form
         attached as Exhibit A hereto, relating to the issuance of one or more
         Certificates of Deposit (the "Certificates of Deposit") by the Cayman
         Islands branch (the "Issuer") of Banco Rio de la Plata S.A. (the
         "Bank") to the Company, the issuance of one or more promissory notes
         (having a principal amount equal to the principal amount of the
         Certificate of Deposit (the "Promissory Notes")) by the Guarantor to
         the Bank and the Guarantees of the Promissory Notes by the Company.

         (b) Section 1.01 of the Indenture is hereby amended by adding a new
clause (ix) to the definition of "Permitted Investment" as follows:

                  (ix) Investments consisting of one or more Certificates of
         Deposit, having an aggregate principal amount not to exceed the
         aggregate principal amount of the Promissory Notes then outstanding;
         provided that (1) upon


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         making any such Investment after the date hereof, the Company shall
         deliver an Officers' Certificate to the Trustee, to the effect that
         applicable law regarding rights of set off has not changed since the
         date hereof, (2) the Stated Maturity of each such Certificate of
         Deposit shall be the same as a Promissory Note of equal principal
         amount and (3) at the time that any Investment in any Certificate of
         Deposit is made the Company shall deliver an Officer's Certificate to
         the Trustee to the effect that (A) the Bank and the Issuer are not
         under intervention, receivership or any similar arrangement or
         proceeding and (B) the Company does not have any reason to believe
         there is a material possibility that the Bank or the Issuer may be
         subject to intervention, receivership or any similar arrangement or
         proceeding.

         SECTION 2. Amendment of Section 4.03(a). Section 4.03(a) of the
Indenture is hereby amended by adding a new clause (xi) to the second paragraph
thereof as follows:

                  (xi) Indebtedness consisting of one or more loans to the
         Guarantor, evidenced by one or more Promissory Notes and Guaranteed by
         the Company, in each case under the Intermediary Documents; provided
         that the Promissory Notes shall, at all times, have an aggregate
         principal amount equal to the aggregate principal amount of the
         Certificates of Deposit and shall not be outstanding at any time that
         the Certificates of Deposit are not validly outstanding and
         beneficially owned by the Company.

         SECTION 3. Amendment of Section 4.04. Section 4.04 of the Indenture is
hereby amended by deleting clause (iv) of the second paragraph thereof and
replacing it with a new clause (iv) as follows:

                  (iv) the repurchase, redemption or other acquisition of
         Capital Stock of the Company or the Guarantor (or options, warrants or
         other rights to acquire such Capital Stock) in exchange for, or out of
         the proceeds of a substantially concurrent offering of, shares of
         Capital Stock (other than Disqualified Stock) of the Company (or
         options, warrants or other rights to acquire such Capital Stock);

         SECTION 4. New Section 4.22. A new Section 4.22 is hereby added to the
Indenture, which Section 4.22 shall read as follows:

                  SECTION 4.22. Protective Actions. The Company shall monitor
         the creditworthiness of the Bank and the Issuer and, if the Company
         shall in good faith conclude that there is any reason to believe that
         there is a material possibility that the Bank or the Issuer may be or
         may become liquidated,


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         insolvent or bankrupt or that any trustee (with respect to the Bank,
         the Issuer or the assets of either of them) may be appointed or that
         the Bank or the Issuer may be subject to receivership or intervention
         or that any similar event may occur, then the Company shall promptly
         take such actions ("Mitigating Actions") as shall be necessary to cause
         the obligations of the Bank and the Issuer under the Certificates of
         Deposit to be promptly satisfied through the cancellation of the
         Promissory Notes and the Guarantees; provided that the Company shall
         not be required to take Mitigating Actions if it has been advised by
         counsel, in writing, that such actions would probably enhance, rather
         than mitigate, the Company's and its Restricted Subsidiaries' exposure
         to credit risk with respect to the Bank and the Issuer.

         SECTION 5. Effectiveness. This Supplemental Indenture shall become
effective on the date duly executed counterparts hereof shall have been signed
by the Company, the Guarantor and the Trustee.

         SECTION 6. Governing Law. This Supplemental Indenture shall be governed
by the laws of the State of New York.

         SECTION 7. Counterparts. This Supplemental Indenture may be signed in
any number of counterparts, each of which shall be an original, with the same
effect as if the signatures thereto were upon the same instrument.

         SECTION 8. Headings. The headings of the Sections of this Supplemental
Indenture have been inserted for convenience and reference only, are not
considered a part hereof and shall in no way modify or restrict any of the terms
and provisions hereof.


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SIGNATURES

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


                                      IMPSAT Corporation


                                      By:
                                         ---------------------------------------
                                         Name:
                                         Title:


                                      By:
                                         ---------------------------------------
                                         Name:
                                         Title:


                                      IMPSAT S.A.


                                      By:
                                         ---------------------------------------
                                         Name:
                                         Title:


                                      THE BANK OF NEW YORK


                                      By:
                                         ---------------------------------------
                                         Name:
                                         Title: