Thalamonal(Innovar) consisted of droperidol, a tranquilizer, and fentanyl, a short acting narcotic. Supplementation of nitrous oxide with large dose of fentanyl was known to cause contraction of abdominal muscle, often strong enough to produce rigidity. There was some reports of interaction of thalamonal on the depolarizing muscle relaxant. This study was undertaken to quantify the neuromuscular effect of thalamonal-N2O compared with halothane-N2O anesthesia on succinylcholine induced blockade. Forty non-pregnsnt women aged between 20 and 40 years undergoing general anesthesia for elective lower abdominal surgery were randomly divided in halothane(20cases) and thalamonal group (20 cases). All were free from renal, hepatic, endocrine and neuromuscular disease. The patient's forarm and hand secured firmly to an armboard, and a force displacement transducer(San Ei, Japan) was connected to patient thumb and ulnar nerve was stimulated through surface elctrodes at wrist. Supramaximal pulse were delivered using a nerve stimulator(Emerson, USA) at a rate of 1 Hz with a duration of 0.2msec. This isometirc twitch tension produced by this stimulation, as quantified by transducer, was continuously recorded before and during anesthesia on a biophysiograph(San Ei, Japan). Anesthesia was induced with thiopental sodium 4~5 mg/kg and succinylcholine 1mg/kg, and intubation was performed after maximal twitch depression. Immediately after intubation, thalamonal 0.5ml/10kg was injected in one group, and halothane 1% was added in the other group. The onset time, time from onset of action to full paralysis, duration of full paralysis and recovery index were analyzed. The results were as follows. 1) In halothane 1% group: the onset time was 16.6±3.21 sec, time from onset to full paralysis was 48.4±8.97 sec and duration of full paralysis was 269.1±61.81 sec. Recovery index was 77.7±19.26 sec. 2) In thalamonal 0.5ml/10kg group: the time to onset of action was 16.0±2.74 sec, time from onset to full paralysis was 43.2±8.09 sec and duration of full paralysis was 347.7±102.03 sec(p<0.05). Recovery index was 93.8±23.86 sec(p>0.05).