The volatile anesthetic agent, halothane and ethrane, are most commonly used in general anesthesia practice and a depolarizing neuromuscular blocker, succinylcholine, is also used for endotracheal intubation, if not contraindicated. There were many reports that the volatile anesthetics affect the neuromuscular transmission in akeletal muscles and potentiate the neuromuscular black induced by depolarizing muscle relaxant in moderate to high concentration. But in halothane, a result was different from those of others. It was well known fact that in pregnant woman, the MAC of the volatile anesthetics is decreased. Therefore, present study was performed to determine whether halothane and ethrane may affect the action of succinylcholine or not, in their low concentration, in 40 healthy pregnant women and we divided them randomly in two groups: halothane administered group(group A), ethrane administered group(group B). Ulnar nerve was stimulated at the wrist through surface electrode, using a peripheral nerve stimulator with supramaximal single twitchimpulse of 0.2 msec duration at a rate of 1.0 Hz. The responses of the adduction of thumb were measured with a force displacement transducer and recorded with a biophysiograph(San Ei, Japan). The single twitch were measured before and after intravenous succinylcholine 1 mg/kg and when full paralysis occurred, 0.5% halothane administered in group A and 1% ethrane administered in group B and observed the effects of halothane and ethrane on the neuromuscular blocking action of succinylcholine. The results were as follows. The time from intravenous succinylcholine onset of paralysis was 15.5±3.21 sec in halothane group, and 14.8±3.17 sec in ethrane group. The time from onset of paralysis to full paralysis was 42.2±5.98 sec in halothane group, and 47.1±13.55 sec in ethrane group. Duration of full paralysis was 283.9±68.02 aec in halothane group, and 270.8±44.49 sec in ethrane group. Recovery index(T26-T76) was 68.4±16.11 aec in halothane group, and 75.4±21.93 sec in ethrane group. We conclude that there is no significant difference between the effects of halothane and ethrane on the neuromuscular blocking actioa of succinylchpline, in their low concentraction, in healthy pregnant women.