The haemodynamic effects of oral prazosin and hydralazine were evaluated in patients with refractory heart failure and compared with those of intravenous nitroprusside in the same patients. Both oral agents were well tolerated and appeared to have beneficial haemodynamic effects. Prazosin and hydralazine produced similar increases in cardiac output associated with a similar decrease in systemic vascular resistance. Prazosin and hydralazine produced similar increases in cardiac output associated with a similar decrease in systemic vascular resistance. Prazosin resulted in a more significant decline in left ventricular filling pressure and pulmonary vascular resistance than did hydralazine. Haemodynamic alterations induced by prazosin were similar to those induced by nitroprusside, which suggests a relatively balanced reduction of preload and afterload. With hydralazine, the increase in cardiac output without change in left ventricular filling pressure or pulmonary vascular resistance suggests minimal effect on preload but significant reduction in afterload.