Because improved long-term oral vasodilator therapy for chronic congestive heart failure is needed, the cardiocirculatory effects of the new antihypertensive quinazoline derivative, trimazosin, were evaluated with use of concomitant cardiac catheterization and forearm plethysmography in nine patients with severe chronic congestive heart failure due to coronary disease. After ingestion of 100 to 300 mg (average 172 mg) of trimazosin, the greatly elevated left ventricular filling pressure decreased from 30 to 23 mm Hg and the lowered cardiac index rose from 2.02 to 2.59 liters/min per m2. Considerable improvement in cardiac function occurred within 1 hour after ingestion of trimazosin; peak efficacy was achieved after 2 hours and persisted in the 3rd hour. Heart rate was unchanged and systemic blood pressure was mildly reduced. Because pump performance was enhanced while indexes of myocardial oxygen consumption declined, ventricular efficiency increased. Vascular relaxation was produced in both the systemic resistance and capacitance beds, with venodilation slightly more prominent. This clinical investigation of the acute hemodynamic effects of trimazosin objectively demonstrates that the drug provides considerable hemodynamic benefit in cardiac dysfunction and is therefore a potentially salutary agent for treatment of patients with chronic severe congestive heart failure.