Alterations in ventricular performance during myocardial ischemia were assessed by relating changes in left ventricular enddiastolic pressures to simultaneous changes in stroke work. Myocardial ischemia, whether induced by exercise or by atrial pacing, was associated with depressed left ventricular function. When angina was prevented by the prior administration of nitroglycerin, ventricular function was normal. Data are presented to indicate the importance of left ventricular volume on the development of angina and to support the concept that a major action of nitroglycerin may be in reducing myocardial oxygen requirements by reducing left ventricular volume. Metabolic studies during pacing-induced angina show a close temporal relationship between the onset of angina, ventricular dysfunction, anaerobic metabolism, and electrocardiographic abnormalities. The prevention of angina by nitroglycerin therapy does not regularly improve the metabolic abnormalities.