In 15 patients with acute myocardial infarction intravenous infusion of the potent vasodilator, nitroprusside, produced an average 50% fall in the elevated left ventricular filling pressure and a modest reduction in arterial pressure (average 15·5%) without a change in heart-rate. Careful dose adjustment in several patients resulted in a significant fall in left ventricular filling pressure with little or no change in arterial pressure. Cardiac output consistently rose during nitroprusside infusion in patients with clinical signs of left ventricular failure or shock and a low control cardiac index, whereas output changed little in those patients with normal control cardiac index. A fall in pressure-time per minute as well as the fall in left ventricular filling pressure indicated a reduction in myocardial oxygen consumption during the vasodilator infusion. Drug infusion was usually accompanied by subjective improvement, including relief of dyspnœa and of chest pain, and reduction in ventricular irritability. These data suggest that vaso dilator therapy, by reducing left ventricular afterload, can improve left ventricular performance in patients with acute myocardial infarction. Such treatment may be a rational approach to the management of patients with heart-failure, pulmonary œdema, or early shock.