Exhibit 5.1

                [LETTERHEAD OF ALSTON & BIRD LLP APPEARS HERE]


                               October 28, 1997


National Data Corporation 
National Data Plaza
Atlanta, Georgia 30329-2010

         Re:  Registration Statement on Form S-4 Covering a 
              Maximum of 1,555,556 Shares of Common Stock
              Registration No. 333-35995
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Ladies and Gentlemen:

    
     This opinion is being rendered in connection with the proposed merger of
Source Informatics Inc. ("Source") with and into a wholly-owned subsidiary of
National Data Corporation (the "Company"), in which the Company will issue up to
1,555,556 shares of its $.125 par value per share common stock (the "Shares"),
upon the terms and conditions set forth in its Registration Statement on 
Form S-4 (the "Registration Statement"), as filed by the Company with the
Securities and Exchange Commission under the Securities Act of 1933, as amended,
on September 19, 1997, as amended.

     As counsel for the Company, we have examined such corporate records and 
documents as we have deemed relevant and necessary as the basis for this 
opinion, and we are familiar with the actions taken by the Company in connection
with the authorization, registration, issuance and sale of the Shares.

     Based upon the foregoing, it is our opinion that the Shares will, upon
their issuance in accordance with the terms and conditions set forth in the
Agreement and Plan of Merger by and among Source, Dunkirk, Inc., and the
Company, dated as of August 20, 1997, be duly authorized and validly issued,
fully paid and non-assessable under the Delaware General Corporation Law as in
effect on this date.

     We hereby consent to the use of this opinion as an exhibit to the
Registration Statement and the reference to our firm in the section entitled
"Legal Matters."


                                Very truly yours

                                ALSTON & BIRD LLP

                                By: /s/ Joel J. Hughey
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