AbstractTo investigate the effects of leg exercise, on the overall tolerance to exaggerated gravitational stress, 8 subjects were exposed for 13 min to a threefold increase of the force of gravity in centrifuge experiments (+ 3 Gz) while in the sitting position, both during motionless resting and when exercising at 600 kpm/min on a bicycle ergometer. In all subjects, continuous leg exercise prevented the occurrence of circulatory collapse, which otherwise became imminent in 5 subjects after 5 to 12 min in the resting condition as evidenced by blackout or rapid increase in heart rate. Subjectively, exercise was experienced as less stressful than the resting condition. The improved circulatory stability with exercise was also reflected by uniformity of individual heart rate responses, the group mean value increasing from 152 to 160 beats/min during the 6th to 12th min.