The objective of this study was to determine the effects of nitroglycerin on ventricular dimensions. At the time of corrective cardiac operations, silver-tantalum markers were sutured to one or both ventricles of 11 patients without clinical evidence of coronary artery disease. Following recovery, cineradiograms were exposed at 30 frames per second and distances between markers were measured before and after 0.6 mg. of nitroglycerin. In all patients, end-diastolic and end-systolic dimensions decreased within 2 to 6 minutes. Right ventricular end-diastolic and end-systolic lengths decreased by an average of 5.0 per cent and 3.6 per cent of control values, respectively, while left ventricular end-diastolic and end-systolic dimensions declined by an average of 6.2 per cent and 5.9 per cent of control, respectively. The diminutions of end-diastolic ventricular dimensions approximated 30 per cent of the stroke volume. Systolic excursions decreased by an average of 13.3 per cent of control for the right ventricle, while left ventricular excursions decreased in four of the six patients. Systolic, mean, and diastolic arterial pressures decreased in each patient after nitroglycerin. Cardiac output fell from an average of 5.6 to 5.0 L./min., decreasing in each of the six patients in whom it was determined, while stroke volume fell from an average value of 65 ml./beat during the control period to 55 ml./beat after nitroglycerin.Thus, nitroglycerin reduces both arterial pressure and ventricular dimensions and in view of the relationship between ventricular size, myocardial tension, and oxygen consumption, this action tends to reduce cardiac oxygen requirements and may explain, at least in part, the effectiveness of the drug in angina pectoris.