To clarify the hemodynamic effects of prazosin in congestive heart failure, the changes of blood distribution were examined radiocardiographically in 9 patients. After obtaining control data, patients were given oral prazosin (5.9 +/- 2.6 mg/day) for 3 weeks. Studies were repeated immediately before and one week after the cessation of the prazosin treatment. Cardiac index (CI) and stroke index (SI) showed significant increases, and mean blood pressure (MBP) and peripheral vascular resistance significant decreases as compared with the control values. However, heart rate did not change significantly throughout the period of this study. After stopping prazosin, MBP, CI and SI returned to the control levels. Concerning the changes in blood distribution, total blood volume and body blood volume significantly increased during prazosin therapy. Pulmonary blood volume (PBV) had a tendency to increase, although the changes were not significant. In patients, in whom the PBV increased during prazosin treatment, diuretics had to be used concomitantly. Our results show that prazosin is effective in the treatment of congestive heart failure and radiocardiography is a useful non-invasive procedure in assessing cardiac improvement and in designing the appropriate treatment.