The effect of nicorandil on the neuromuscular block induced by vecuronium was investigated in vitro with rat phrenic nerve-hemidiaphragm preparation. Both single twitch (ST) and train-of-four ratio (TOFR) elicited with indirect electrical stimulation were used. The administration of nicorandil alone showed no effects on ST and TOFR. The pre-administration of nicorandil exerted significant depressing effect on ST compared with vecuronium alone. High concentration of nicorandil (4 x 10(-6) M) produced more significant effect on ST compared with low concentration of nicorandil (10(-7) M). In this study, the nicorandil showed similar effect as calcium antagonists on neuromuscular blockade.