The neuromuscular blocking effect of pipecuronium was evaluated in 35 patients under N2O-O2-isoflurane anesthesia with visual and/or tactile counts for the twitch of the adductor pollicis muscle in response to train-of-tour(TOF) stimulation of the ulnar nerve at the wrist. Group I, II and III were classified according to the initial dose of pipecuronium of 50, 80 and 100ug/kg, respectively. The additional dose, 30 ug/kg, was given in all three groups when the first twitch of TOF(T) reappeared. The onset time in Group I, II and III was 361.4+/-98.6, 218.7+/- 80.8 and 239.0+/-73.7 seconds, respectively. The onset time in Group I was significantly slower(p<0.005) than those in the other groups. All three doses of pipecuronium provided good to exceUent intubating condition in about 4 to 6 minutes after the administration of the initial dose. The time interval from the disappearance of T1 to the reappearance of T1 was 39.0+/-20.8 min in Group I, which was significantly longer(p<0.05) in Group II(67.7+/-26.4 min) or III(63.8+/-20.8 min). The cumulative effect of pipecuronium was evaluated by comparing the mean time intervals of an additional dose to the succeeding ones. The intervals between additional doses were independent of the size or duration of the initial dose. There were no significant differences in the intervals between additional doses. Heart rates, rhythms and mean arterial blood pressures were not significantly changed in any groups following the administration of pipecuronium In conclusion, pipecuronium bromide can be recommended as a long-acting neuromuscular blocking agent with an absence or minimum of cumulative and cardiovascular effects for patients in whom a long operation is scheduled and the cardiovascular stability is required.