For the assessment the effect of succinylcholine (SCh) on pipecuronium, 52 adult patients undergoing elective surgery under general anesthesia were subjected to this study in which the EMG response (twitch height of the hand to TOF stimulation with 2Hz) of ulnar nerve was monitored and recorded with Datex Relaxograph. According to the amount and mode of the drugs administered, the patients were divided into four experimental groups: 1) Group I: a bolus injection of pipecuronium in dose of 0.05 mg/kg. 2) Group II: pipecuronium 0.1 mg/kg, a double dose of group l. 3) Group IU: pipecuronium 0.05 mg/kg given when the depressed twitch height by SCh (1 mg/kg) recovered to 25%6 of initial twitch height. 4) Group IV: mixed injection of SCh (1 mg/kg) and pipecumnium (0.05 mg/kg). The results were as follows ; 1) Mean onset time of pipecuronium was 6.5±0.5 minutes in group I and 4.1±0.5 minutes in group II, the latter being significantly shorter than group I (p<0.01). In group Ill, it was 2.1±0.23 minutes being significantly shorter than group I, II (p<0.001). In group IV it was 1.1±0.1 minutes which was more significantly shorter than group I, II, and IU. 2) Mean action duration of pipecuronium was 50.9±6.7 minutes in group I and 141.9±15.4 minutes in group II, the latter being longer significantly (p<0.001). In group IIl, it was 53.9±5.2 minutes which was similar to group I, but it was 69.8±6.5 minutes in group IV, being significantly longer than those of group I and III (p<0.05). 3) Mean potency of pipecuronium expressed by the percentage change of initial twitch height was 7.6±1.9% in group I, but it was significantly decreased to 4.2±0.9% in group II (p<0.05). In group III, it was 0.2±0.1% being sinificantly decreased than group I, II (p<0.001). In group IV, it was 0.0+0.0% being more significantly decreased than other groups (p<0.001). 4) Presence of pipecuronium in group IV did not affect on the intensity of fasciculation induced by SCh. These results indicate that succinylcholine may potentiate the pipecuronium based on the findings that succinylcholine increased the potency and lengthened the duration of action of pipecuronium.