Hydralazine was administered in a single oral dose of 50 to 100 mg in 16 patients with left ventricular failure due to cardiomyopathy. It produced sustained effects for at least 4 h characterized by a significant increase in cardiac output, a reduction in arterial and pulmonary arterial pressure, and a slight rise in heart rate. When compared to nitroprusside infusion in these same patients, hydralazine produced a similar reduction in systemic vascular resistance but a slightly greater increase in cardiac index (0.74 versus 0.95 litres/min.m2), with a lesser fall in mean arterial pressure (7.8 versus 13.6 mm Hg, P<0.01), mean pulmonary arterial pressure (4.2 versus 11.3 mm Hg, P<0.001), and pulmonary wedge pressure (5.5 versus 9.9 mm Hg, P<0.001). Forearm venous tone decreased and venous compliance increased during nitroprusside infusion, but they were unchanged after hydralazine therapy. These data suggest that hydralazine may be a useful agent in the treatment of chronic left ventricular failure.