A new oral vasodilator, prazosin, was evaluated in twenty patients with refractory congestive heart in a double blind non cross-over trial. Patients were divided into two groups. The first (group A) was treated by prazosin (40-50 microgram/Kg/pro die) for 20 days, the second was given placebo for the same period. Ventricular performance was assessed by measurement of the systolic time intervals, the peak time of the first derivative of the apexcardiogram (delta ACG), pressure-rate product and clinical evaluation. After ten days of prazosin left ventricular ejection time (LVET) increased (P < 0.01), pre-ejection period (PEP) and relation PEP/LVET were reduced (respectively: P < 0.05 and P < 0.01). Similarly the peak of delta ACG and pressure-rate product decreased significantly (respectively: P < 0.05 and P < 0.01). Symptomatology improved in NYHA functional class (3.7 to 2.4, P < 0.001). After twenty days of prazosin there was no further significant improvement in myocardial performance. No significant changes of clinical course and of the same parameters were observed in patients of group B. It is concluded that prazosin is effective in improving cardiac performance in refractory heart failure for its balanced dilator action on the arteriolar and venous vessels.