On April 3, 1965 the world's first nuclear reactor space power system, SNAP I0A, was launched and operated successfully for forty-three days in orbit. To reduce and analyze the flight test data, data handling procedures and data analysis computer programs representing the end results of three years of effort and improvement of similar data systems for SNAP 10A ground tests were used. This paper describes the evolution of data handling and reduction from the manual methods used for the first non-nuclear system test to later ground tests where data loggers of increasing degrees of sophistication were used to generate punched paper tape which was then computer processed, reduced, and analyzed, and finally to the completely electronic and computerized reduction of flight test telemetry data. The equipment used, handling procedures developed, and computer programs are briefly described. Improvements in each generation of data system resulting from experience and improved equipment are emphasized. As a result of the experience in semi-automatic data collection, reduction, and analysis gained in the SNAP 10A program recommendations for creation and improvement of similar data systems are given, and some of the more important pitfalls encountered are discussed. Finally, the more significant differences between flight and ground acquisition and reduction are noted.