The haemodynamic effects of separate and simultaneous nitroglycerin (NTG)--and dobutamine (DOB) infusions were determined in order to evaluate the nature of the effects of combined treatment. Ten patients were studied, 6 h after coronary by-pass surgery. NTG alone reduced significantly systemic and pulmonary arterial, as well as ventricular filling pressures, and brought about a concomitant 10% decrease in the cardiac index (P < 0.05). Dobutamine (6 microgram/kg/min) did not affect systemic or pulmonary arterial pressures, nor ventricular filling pressures. As a result of the 20 beats/min rise in heart rate produced by DOB, the cardiac index was increased by 24%, whereas the stroke volume and work indices remained unchanged during DOB-infusion. The haemodynamic changes brought about by either NTG or DOB separately were unaffected by the simultaneous infusion of the other vasoactive drug. The haemodynamic effects of the combined treatment were simply the sum of their separate effects, thereby yielding reduced arterial and ventricular filling pressures, increased heart rate, and slightly augmented cardiac index with significantly reduced systemic vascular resistance; myocardial oxygen consumption was unaffected by the combination therapy. One patient, whose case is discussed separately, had a hypotension-bradycardia complication during NTG-infusion.