Vasodilators used in chronic congestive heart failure are not optimal in that nitrates are predominant venodilators, prazosin is associated with tolerance development, and hydralazine produces chronic toxicity. Therefore, we studied the acute hemodynamic effects of a single dose of minoxidil in 18 patients with chronic left ventricular failure caused by ischemic or primary cardiomyopathy. Peak effects were observed 5 hours after single oral doses of minoxidil, averaging 15.3 ± 1.4 mg (SEM). Heart rate rose slightly, from 85.4 ± 2.9 to 90.0 ± 3.2 beats/min, after minoxidil (p < 0.02) and mean arterial pressure fell slightly, from 88.0 ± 2.3 to 84.9 ± 2.5 mm Hg (p < 0.05). Cardiac index increased from 2.34 ± 0.14 to 2.95 0.29 l/min/m2 after minoxidil (p < 0.02) and systemic vascular resistance fell from 19.6 ± 1.5 to 15.0 ± 1.3 units (p < 0.01). Minoxidil did not affect right atrial, pulmonary arterial and pulmonary wedge pressures. Hemodynamic effects of minoxidil persisted for at least 8 hours after a single dose. Minoxidil appears to be an effective arterial dilating agent in patients with heart failure and resembles hydralazine in its actions. Because of its potency, prolonged duration of action and relatively low toxicity, minoxidil may be a useful vasodilator for heart failure. However, its long-term effect must be further evaluated.