Physiological responses of athletes and nonathletes to treadmill exercise at 4 mph on a 10% grade for 5 min were compared. Some nonathletes were studied again after 4 weeks' physical training. Whereas the increase in oxygen consumption during exercise and the oxygen debt were similar in all groups, heart rate and blood flow in the leg increased less and returned to resting values more rapidly in athletes and in trained nonathletes than in untrained nonathletes. The time required for the return of blood flow to pre-exercise levels was greater than that necessary for the repayment of O2 debt. Circulatory changes involved in body temperature regulation during exercise had little effect on total leg blood flow. Experiments with a foot pedal exercise, with and without occlusion of circulation to the leg, suggest that athletes and trained nonathletes have greater blood flow in the muscles during exercise than do sedentary subjects. In the latter, the first few minutes after exercise without occlusion resembled recovery from exercise with occlusion, indicating that relative ischemia is normal in untrained subjects during moderate exercise. Submitted on October 19, 1961