Anesthetic drugs by themselves were without effect on neummuscular function, but when the margin of safety in transmission was reduced, because of the admimstration of a musde relaxant, then a synergistic effect on relaxation might be observed. The purpose of this study was to examine whether the twitch recovery after vecuronium bromide (vecuronium) was influenced by thiopental sodium (thiopental), ketamine and lidocaine. Eighty healthy adult patients were randomly allocated to four groups according to the drugs, control group, normal saline 5 ml, study group ; 2.5% thiopental 5 mg/kg, ketamine 2 mg/kg and 1% lidocaine 1 mg/kg. Neuromuscular bloek was induced by intravenous vecuronium 0.1 mg/kg and applied ulnar nerve stimuli of every 10 seconds single twitch using Myotest Mk II(R). The adduction force of the resultant thumb twitch was measured by the acceleration of a small piezo-electric ceramic wafer with electrodes of Mini-accelograph(8) and recorded by Datascope 2200I(8). When the twitch height was near 15 mm, the drug was injected and recorded the twitch height for 10 min. and checked mean arterial pressure and heart rate after injection. The results were as follows ; 1) There were no significant changes of mean arterial pressure after drug injection. 2) Heart rate was significantly increased in thiopental group (77.7+/-8.3 beats/min. 93.7+/-9.1 beats/min.) and lidocaine group (85.6+/-15.9 beats/min. - 88.4+/-16.4 beats/min.), but no change in control and ketamine group. 3) In the linear regression analysis between the twitch length and the time, control group was Y=16.0+/-1.0X, r2=0.33, thiopental group was Y=14.9+/-1.1X, r2%.39, ketamine gmup was Y=14.8+0.9X, r2=0.24 and lidocaine group was Y=15.1+0.8X, r2=0.30. Lidocaine group was only significantly decreased than control group. With the above results the authors concluded that the twitch recovery after vecuronium was significantly deaeased by lidocaine, but no cbanges by thiopental and ketamine. We should more find the drug interaction with vecuronium in the man.