The physiologic effects of systemic vasoconstriction on left ventricular performance and the salutary hemodynamic effect of acute administration of vasodilator drugs to patients with heart failure provide a rational basis for vasodilator therapy in acute myocardial infarction and chronic congestive heart failure. Use of vasodilators during the acute phase of myocardial infarction may reduce mortality when left ventricular filling pressure remains markedly elevated for at least 8 hours after the onset of clinical syndrome. Use of chronic vasodilator therapy in patients with congestive heart failure appears to have been effective in some patients in prolonging exercise tolerance; however, the likelihood of a beneficial effect and the impact of this therapy on the natural history of the disease remain unclear. A Veterans Administration Cooperative Study to address these questions is currently in progress. The new effort directed to studying the effects of vasodilator drugs in these syndromes has appropriately focused attention on their pathophysiology and natural history.