Since sympathetic stimulation could play an important part in the short term attacks of angina pectoris, the effects of nitroglycerin on the myocardial response to levarterenol were investigated. Levarterenol infusions directly increased the isometric systolic tension (IST) developed by the cardiac muscle. Nitroglycerin had no apparent direct effect on this increase in IST either in the normal heart or in the heart following partial coronary ligation. In experiments in which IST was measured with a lever system adjusted to the end-diastolic length and tension occurring just prior to the contraction, nitroglycerin was found to have an indirect depressant effect on the levarterenol response. This decrease in contractility was shown to be secondary to a decrease in hemodynamic work load elicited by nitroglycerin. It is suggested that the chief effect of nitroglycerin in the relief of angina pectoris is due to this agent's ability to decrease a work load imposed on the myocardium by sympathetic stimulation.