Maintenance of rest and exercise performance are prime requirements for all astronauts during flight, not only for maintaining day-to-day productivity, but also for coping with unlikely emergencies; e.g., emergency egress from the Shuttle upon landing. Indirect estimates of submaximal work capacity (oxygen uptake) made from changes in the heart rates of 27 Apollo astronauts (<15-day flights) indicated a reduction in work capacity (maximal oxygen uptake) of 17% to 21%. This percentage decrease was similar to that measured in middle-aged men after 21 to 30 days of -6° head-down bed-rest deconditioning without exercise training. Heart-rate changes during submaximal exercise in the nine Skylab astronauts suggested that they were better able to maintain their work capacity because of longer and more intensive in-flight exercise training. The strength of the flexor and extensor muscle groups decreased by 2% to 9% in the elbow and by 6% to 20% in the knee in the Skylab astronauts, also similar to the decreases in men undergoing 30 days of bed-rest deconditioning. Exercise protocols have been devised that result in maintenance of work capacity and muscular strength during 30 to 49 days of bed-rest deconditioning. The major question is whether maintenance of strength and work capacity at normal 1-G levels is necessary during prolonged spaceflights.