There is abundant experimental proof that the xanthines theobromine and theophylline, as well as other members of the series, are potent vasodilators of the coronary arteries. These drugs have been shown to possess this property in human1 and canine2 heart-lung preparations: in the intact anesthetized dog by measuring the outflow from the coronary sinus,3 and in the intact anesthetized and trained conscious dog by using a thermostromuhr4 on one or another of the coronary arteries. Levy and his associates5 have demonstrated this property of the xanthines inferentially and reported the favorable influence of these drugs on patients with attacks of angina pectoris induced by breathing an atmosphere containing only 10 per cent oxygen. In a recent report Gilbert, Fenn and LeRoy6 have confirmed this observation. Experiments soon to be published by Gilbert and me demonstrate that the mortality after experimental occlusion of the coronary