The neuromuscular blocking effsct of mivscurium 0.15mg/kg and atracurium 0.5mg/kg were compared in 17 adult patients (ASA class I and II) during N2O propofol-fentanyl anesthesia. Neuromuscular transmission was monitored by recording the compound electromyogram of the hypothenar muscles resulting from supramaximal train of four stimulation applied to the ulnar nerve. Neuromuscular block was recovered spontaneously in all cases. Onset time of over 95% block was not different significantly between mivacurium and atraeurium (201±37.6vs 176.6±28.5sec, p>0.05, mean SD). Duration from injection of mivacurium to 25% recovery of control twitch height was significantly shorter than that of atracurium (13.0±2.2vs 38.8±4.9min, p<0.05). The recovery index was also significantly shorter for mivacurium than for atracurium (6.9±1.1vs 14.5±53.0min, p<0.05). In conclusion, we have confirmed that duration and recovery of action of mivacurium were shorter than those of atracurium, but onset times of both agents were not different.