Alterations in the physiology of the cardiovascular system have been noted during all exposures to the microgravity experienced in space flight. Of most importance to the operational function of Space Shuttle crewmembers is orthostatic intolerance. Although complex changes occur as a result of adaptation to weightlessness, the redistribution and loss of body fluid apparently plays a substantial role. Utilizing ground-based bed rest data as an analog to the absence of gravitational force encountered in orbital flight, a saline loading countermeasure was developed. In this study, 17 crewmembers consumed various amounts of salt and fluid prior to the reentry phase of Space Shuttle flights; 9 other astronauts served as control subjects. The countermeasure reduced the heart rate response to orthostatic stress 29% and reversed the fall in mean blood pressure. A Cardiovascular Index of Deconditioning (defined as CID = delta HR - delta SBP + delta DBP) equalled 21 in those who utilized the countermeasure, a significant improvement toward baseline (p less than 0.003) when compared to the control group CID = 49. The encouraging results of these investigations have led to the adoption of the countermeasure as an operational procedure by Shuttle crewmembers.