The long-term effects of prazosin in chronic congestive heart failure were studied in 10 patients (New York Heart Association class III-IV) in a double-blind cross-over study. Patients with systolic blood pressure greater than 120 mmHg and left ventricular filling pressure greater than 15 mmHg were included. Prazosin lowered the arteriovenous oxygen difference both at rest and during exercise (p less than 0.05), increased cardiac index (p less than 0.01) and reduced right atrial pressure and systemic vascular resistance (p less than 0.05) during exercise. Left ventricular filling pressure was also reduced, but not significantly, during exercise. Our data show that prazosin has beneficial long-term effects during exercise in patients with chronic congestive heart failure.