Nicorandil is a hybrid of a nitrate and a potassium channel opener, and has a potent vasodilatory effect on coronary arteries. The effects of intracoronary injection of nicorandil on coronary circulation were examined in 12 adult patients with angiographically normal or near-normal left coronary arteries to determine the optimal dose of this agent. The intracoronary injection of nicorandil (up to 1 mg over 1 min) dilated left coronary artery segments in a dose-dependent manner, with no significant effects on systemic hemodynamic parameters. The percent increase in the epicardial coronary artery diameter with 1 mg nicorandil 31±5%, mean±SEM) was not significantly different from that with 0.3 mg sublingual nitroglycerin (39±55%). Coronary sinus venous oxygen saturation increased immediately after the intracoronary injectin of 1mg nicorandil, and then returned to the baseline level within 3 min. Neither arrhythmias nor conduction disturbances were observed. These results indicate that dilatation of the epicardial coronary artery was achieved with intracoronary nicorandil in a dose-dependent manner (up to 1 mg over 1 min) without any adverse effects in man, and the dilatory effect on coronary resistance vessels was of short duration.