It has been known that exercise causes an increase in the coronary blood flow in animals. The present work has been carried out to study the effect of exercise on coronary blood flow and myocardial oxygen consumption of the human heart in vivo. The results indicate that the heart responds to the increased load of exercise with a rise in coronary blood flow. Since the arteriovenous coronary oxygen difference shows little change, the increase in oxygen consumption of the heart muscle is primarily the result of an increased coronary blood flow. As the cardiac work rises more than the myocardial oxygen consumption, the left ventricular efficiency increases. The response of the failing heart muscle to acute increases in load produced by exercise does not differ from that of the normal heart or of the isolated heart. C ATHETERIZATION of the coronary sinus in man in conjunction with the nitrous oxide method has made it possible to determine coronary blood flow and myocardial oxygen consumption in man.1'2 Until now, the method has been used to follow the behavior of human heart muscle of resting individuals only.3 A study of the coronary circulation and myocardial oxygen consumption during exercise would appear to be of interest, because exercise constitutes a temporary increase in load to which the heart must adjust itself by coronary circulatory and metabolic changes. The alterations in the coronary circulation and myocardial oxygen consumption are of particular importance since it is during in-

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