To determine the possible modifying influence of potent inotropic medication and of sodium nitroprusside on the haemodynamic effects produced by each other, ten patients were treated 4 h after open-heart surgery with constant infusions of nitroprusside (NP) and dobutamine (DOB), separately and in combination. In consequence of reduced left ventricular filling pressure, NP produced, the cardiac index fell tolerably, although systemic and pulmonary vascular resistances were typically lowered. These haemodynamic effects of NP were identical, with and without simultaneous DOB-infusion. Dobutamine (6 microgram/kg/min) did not affect systemic or pulmonary arterial mean pressures, nor the filling pressures of right or ventricle. It augmented the cardiac index by 18%, but as a result of a 17 beats/min rise in heart rate, the stroke volume remained unchanged during DOB-infusion, whereas systemic vascular resistance was significantly lowered. The cardiovascular responses to DOB were independent of the simultaneous NP infusion. To conclude, the haemodynamic changes brought about by NP or DOB are not affected by each other, but when combined they produce additive, beneficial haemodynamic effects. Myocardial oxygen consumption, reflected as the rate-pressure-product (R-P-P), was slightly reduced during the combination therapy, due to the summation of the marked decrease of R-P-P which NP produced and the moderate increase of R-P-P produced by DOB.