In a placebo-controlled trial, we have studied the vasodilator properties of bolus dose of nitroglycerin, isosorbide dinitrate and chlorpromazine in 38 patients during cardiopulmonary bypass with a constant pump flow. Mean arterial pressure and blood volume of the venous reservoir were recorded for 10 min after drug administration to detect changes in arteriolar resistance and venous capacitance, respectively. Nitroglycerin, 2.5 ug/kg, decreased arterial pressure, but the effect lasted for 3 minutes. Chlorpromazine, 0.1 mg/kg, decreased arterial pressure for 9 minutes. Isosorbide dinitrate, 20 ug/kg, had no significant change on arterial pressure. The venous capacitance-increasing effects of nitroglycerin and chlorpromazine were significant for 4 minutes after the bolus. Thereafter the effect of nitroglycerin began to decline, while that of chlorpromazine significantly continued. Isosorbide dinitrate had no significant change on venous reservoir level. The SVR reduction effects of nitroglycerin was significant for 3 minutes, chlorpromazine decreased SVR for over 10 minutes. In conclusion chlorpromazine effect on arterial pressure and venous capacitance was more potent and longer than nitroglycerin and isosorbide dinitrate. Nitroglycerin and chlorpromazine effect on preload and afterload were significant after bolus dose.