The Viking project is managed by the Viking project office at Langley Research Center for the National Aeronautics and Space Administration. Two Viking spacecraft will be individually launched on a new Titan IIIE/Centaur D-1T launch vehicle in August 1975.The method for the establishment of spacecraft sine vibration test levels prior to availability of any Titan IIIE/Centaur D-1T flight data by use of both computer simulations and data from previous Titan and Atlas Centaur vehicles is described. The specification level is compared with actual flight data obtained from a proof flight launch of the Titan IIIE/Centaur D-1T and a Viking dynamic simulator in January 1974. An objective of the proof flight launch was to obtain estimates of the flight loads and environments.The criteria used to minimize the structural weight that would result from an unmodified application of a sine test environment are described. During the sine test, the loads in the primary structural members are limited to values based on computed flight loads. The frequency range of load limitation is directly dependent on the accuracy of the loads analysis procedure.