Train-of-four (TOF) fade is a characteristic of the nondepolarizing neuromuscular blocking agents, but the degree of TOF fade has been shown to vary for different nondeplarizing relaxants. In the present study we measured the degree of fade of four nondepolarizing muscle relaxantsatracurium, vecuronium, pancuronium, and d-tubocurarine-during spontaneous recovery of neuromuscular blockade. Forty ASA class 1 or 2 adult patients were studied. All patients were premedicated with glycopyrrolate 0.2 mg and nalbuphine 10 mg IM, 1 hour before induction. After induction with pentothal sodium 5-6 mg/kg IV, neuromuscular monitoring was carried out by stimulaton of the ulnar nerve at the wrist with supramaximal stimuli at a frequency of 2 Hz repeated every 20 seconds using an ABM (Anesthesia and Brain activity Monitor, Datex Co.). The electromyographic response of hypothenar muscles was recorded. Following stabilization of control twitch height, the nondepolarizing relaxant was injected as a bolus dose, and tracheal intubation was performed when the first twich height of the train (T(1)) was 5% of the control. Anesthesia was maintained with O2-N2O (50%) -enflurane. Patients were divided into four groups according to use of relaxant: atracurium 0.4 mg/kg (n=10: group A), vecuronium 0.08 mg/kg (n=10; group V), pancuronium 0.08 mg/kg (n=10; group T), and d-tubocurarine 0.4 mg/kg (n= 10: group T). T(4) ratios at T(1) of 25, 50 and 75% recovery of T(1) and recovery index in each group were measured. The results were as follows: 1) The T(4) ratio was decreased significantly in P and T groups, as compared to A and V group. There were no significant differences between P and T groups, and between A and V groups (except 25% recovery of T(1)). 2) The recovery index was 14.8+/-2.36 min in A group, 16.4+/-1.85 min in V group, 51.2+/-4.17 min in P group, and 93.4+/-7.31 min in T group.