Twenty patients with congestive heart failure underwent hemodynamic studies before and over 10 hours after the administration of 25, 50, and 75 mg of indoramin, an alpha 1-adrenergic antagonist. Hemodynamic studies were repeated during exercise after the administration of the optimal dose of indoramin. The drug reduced resting and exercise pulmonary capillary wedge pressure, right atrial pressure, systemic blood pressure and vascular resistance, and pulmonary artery pressure and vascular resistance. Resting and exercise stroke volume and cardiac output rose in response to the fall in vascular resistances. Heart rate was not altered at rest or during exercise. The first dose of the alpha 1 blocker indoramin elicits a significant reduction in ventricular preload and afterload and augmentation of ventricular performance in patients with congestive heart failure.