OPC-18790, (+/-)6-[3-(3,4-dimethoxybenzylamino)-2- hydroxypropoxy]-2(1 H)-quinolinone. is a novel positive inotropic agent with a moderate vasodilating action. We examined the vasodilating action of OPC-18790 in detail in the pentobarbital-anesthetized dogs using a colored microsphere technique for resistance vessels and a mean circulatory filling pressure method for capacitance vessels. Intravenously (i.v.) infused OPC-18790 increased the first derivative of left ventricular pressure (LVdP/dt max), cardiac output, heart rate and decreased total peripheral resistance but did not affect mean blood pressure. OPC-18790 significantly increased arterial blood flow distribution to heart and decreased vascular resistance in heart. OPC-18790 at 300 mu g/kg i.v. and nitroglycerin at 50 mu g/kg i.v. did not affect mean circulatory filling pressure in intact anesthetized dogs, but both compounds decreased mean circulatory filling pressure in spinally anesthetized dogs. OPC-18790 also decreased resistance to venous return but nitroglycerin did not. These results suggest that OPC-18790 has both arterial and venous vasodilating actions in addition to its positive inotropic action. These actions may produce an improvement of cardiohemodynamics in heart failure.