The human cardiovascular adaptation to microgravity was investigated in the framework of the German Spacelab D2 mission. Preflight and postflight studies were performed to examine the relationship between disuse atrophy and the function of cardiac and skeletal muscles. Special attention was given to fluid load responses and postflight orthostatic hypotension. The preflight measurements were obtained, in supine and sitting positions. These measurements, carried out in the four D2 crew members, were performed six and nine months before flight and on mission day number five. The results obtained on the male crew showed that the stroke volume data from microgravity are virtually identical to preflight measurements in the sitting position.