In 31 canine heart-lung preparations the direct cardiac effects of dopamine were compared to those of norepinephrine and orciprenaline. All three agents increased heart rate by 50-60% to a maximum of 240-250 beats/min. The inotropic effects of the three drugs were not significantly different. Even with the highest doses cardiac arrhythmias were rarely observed. To achieve maximal chronotropic and inotropic effects, dopamine required four times higher doses than norepinephrine and orciprenaline. All three drugs increased right ventricular dP/dt max by about twice as much as left ventricular dP/dt max. Peak systolic pressure (P max) in the right ventricle was increased 4-5 times more than P max in the left ventricle while mean aortic pressure was elevated by 12-16 mm Hg and mean pulmonary pressure by 6-14 mm Hg. The positive inotropic effect of all three sympathomimetics and of CaCl<sub>2</sub> was substantially more pronounced on the right than on the left ventricle.