Dopamine was administered intravenously to two groups of anesthetized dogs at rates of 2.5, 5, 10, and 20 μg/kg/min. After the 20 μg/kg/ min infusion, mean arterial pressure, heart rate, renal blood flow, mean pulmonary arterial pressure, and left ventricular filling pressure were increased. The dopamine infusion was continued in both groups of animals. In Group I phentolamine was administered at rates of 2.5, 5, and 7.5 μg/kg/min; in Group III sodium nitroprusside at rates of 1, 2, 3, and 4 μg/kg/min. Phentolamine and sodium nitroprusside produced similar reductions in mean arterial blood pressure, mean pulmonary arterial pressure, left ventricular filling pressure, and similar increments in heart rate. Renal blood flow was maintained with phentolamine infusions and decreased slightly with the nitroprusside infusions. These studies demonstrated that both phentolamine and nitroprusside have the ability to reduce the vasoconstricting effects of dopamine.