Vasodilating effects of intravenous administrations of nicorandil (SG-75) and nitroglycerin were analyzed in anaesthetized open-chest dogs by measuring simultaneously, and continuously, coronary (CBF), vertebral (VBF), renal (RBF) and aortic blood flow (AoF). Nicorandil 10-300 micrograms/kg i.v. and nitroglycerin 1-30 micrograms/kg i.v. decreased aortic blood pressure and increased CBF in a dose-dependent fashion. The doses of nicorandil and nitroglycerin which reduced coronary vascular resistance to about 60% of the predrug value were 100 micrograms/kg and 10 micrograms/kg, respectively. Nicorandil 100 micrograms/kg i.v. significantly increased AoF and heart rate, significantly decreased left ventricular end-diastolic pressure and did not significantly change VBF, RBF and left ventricular dP/dt. Nitroglycerin 10 micrograms/kg i.v. significantly increased VBF and heart rate, significantly decreased left ventricular end-diastolic pressure and produced an initial increase followed by a decrease in AoF and RBF. When compared with these doses of both drugs, the ratio of percent decrease in coronary vascular resistance to that in total peripheral resistance was over 1.0 in both drugs and the value of this ratio in nicorandil was significantly larger than that in nitroglycerin. The duration of increase in CBF produced by nicorandil 10-300 micrograms/kg i.v. was dose-dependent, but was not changed by nitroglycerin 1-30 micrograms/kg i.v. The results indicate that nicorandil and nitroglycerin dilate coronary vasculature more markedly than other vascular beds and that the potency of selective coronary vasodilatation and the duration of action are more significant in nicorandil than in nitroglycerin.