The hemodynamic effects of dobutamine, dopamine, and a combination of norepinephrine-phentolamine were compared in nine patients with low cardiac output state after open heart surgery. Using a Latin square design for drug sequence, each patient was given each drug at three levels by continuous intravenous infusion. For dobutamine and dopamine the doses were 2.5, 5.0 and 10.0 microgram/kg/min; for norepinephrine, 0.02, 0.05 and 0.10 microgram/kg/min with phentolamine in the ratio of 1 (norepinephrine) to 2.5 (phentolamine). Dobutamine and dopamine both produced an increase in heart rate, mean arterial pressure, cardiac index, and stroke volume index. Both drugs reduced total pulmonary and systemic vascular resistance in the dose range of 5.0 to 10.0 microgram/kg/min. When the maximum tolerated doses were compared, the drugs equally increased cardiac index and stroke volume index, but dopamine did so at a smaller dose than dobutamine. At equivalent doses, dobutamine tended to produce less tachycardia and vasoconstriction than dopamine. The norepinephrine-phentolamine combination resulted in increases in both systemic arterial pressure and vascular resistance with no significant increase in cardiac index. Its unique feature was its predictable vasopressor response with absence of chronotropic effects and ventricular irritability.