Many articles have appeared recently in the lay press extolling the virtues of muscular exercise. Further, experiments in our laboratory have indicated that after occlusion of a major blood vessel, regular, progressive muscular exercise seems to speed the development of collateral circulation. Clinically, our experience is similar, for patients with claudication from segmental arterial occlusion may increase their walking tolerance by progressive daily exercise. Thus, since there are forces existing which tend toward increasing muscular activity in persons who may be atherosclerosis-prone, it has become important to know whether exercise may retard or augment atherosclerosis. We have tried to secure information on this point, using cholesterol-thiouracil-induced atherosclerosis in dogs and recognizing full well the objections to such preparations. Methods Ten mongrel dogs of similar size and weight were placed on a diet of 70% horsemeat, 25% feed grain, 5% cholesterol, and 150 mg. of thiouracil daily. One pound of this