In order to evaluate the possible differences in response to two currently preferable inotropic-vasodilator drug combinations, the hemodynamic effects of automated infusions of nitroprusside-dobutamine and nitroprusside-dopamine were studied in two clinically similar groups of patients with low-output syndrome after aorto-coronary bypass surgery. Nitroprusside, when infused in combination with dopamine, was effective in lowering filling pressures of both ventricles and systemic and pulmonary arterial pressures reflecting the reversal of deleterious rise of these parameters including determinants of myocardial oxygen consumption (mean arterial pressure rate by +6%, heart rate by +20%) caused by dopamine (6 micrograms/kg/min) infused alone. The combination of the more cardioselective catecholamine dobutamine with nitroprusside brought about similar increase in cardiac output by 50%, but derived its hemodynamic efficiency from the sum of separate effects of components and produced less in the way of elevation of determinants of oxygen consumption. Dobutamine when infused alone (6 micrograms/kg/min) has not affected systemic and pulmonary arterial pressures and stroke index rose more markedly (+9%) as a result of more moderate heart rate increase (+13%). Nitroprusside contributed in an additive manner to the achieved overall reduction of SVR and PVR by 43% and 50%, resp., the automated infusion being helpful in maintaining these values within close tolerances.