Background : Although the neuromuscular blocking properties of neostigmine have been known for many years, few assessment of this effect has been described in man. The aim of this study is to determine the effect of neostigmine on neuromuscular transmission in the absence of muscle relaxants. Methods : ASA I, twenty unpremedicated patients who had given their informed consent were administered neostigmine 2.0 mg and atropine 0.5 mg under propofol, fentanyl, and O2-N2O(67%) anesthesia. Train-of-four responses were measured electromyographically using 0.2 msec, 2 Hz of supramaximal ulnar nerve stimulation every 20 sec. Results : There was no alteration in the train-of-four ratio, but the first twitch height decreased significantly after administration of neostigmine(p＜0.05). Fasciculations were noted in the face and arms. Heart rate was decreased significantly to compare with preinjection value(p＜0.05) while mean arterial pressure was not changed. Conclusions : It is concluded that even in clinical doses neostigmine may produce a depolarizing neuromuscular blockade, probably from the consequence of excessive accumulation of acetylcholine in the neuromuscular junction.(Korean J Anesthesiol 1997; 32: 940∼945)