For the assessment the effect of succinylcholine (SCh) on atracurium, 59 adult patients undergoing elective surgery under general anesthesia were subjected to this study in which the nEMG response (twitch height) of the hand to TOF stimulation (0.2 Hz) 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 atracurium in dose of 0.5 mg/kg 2) Group II, atracurium 1.0 mg/kg, a double dose of group I 3) Group III, atracurium 0.5 mg/kg after 25 to 50% recovery of initial twitch height from twitch height depression induced by SCh (1 mg/kg) 4) Group IV, mixed injection of SCh (1 mg/kg) and atracurium (0.5 mg/kg) Followings were the results. 1. Mean onset time of the atracurium, which was 4.5+/-0.6 minutes in group I and 4.00.7 minutes in group II, changed to 2.7+/-0.6 and 4.0+0.8 minutes in group III and IV respectively, showing no significance. 2. Mean action duration of atracurium was 49.1+/-3.4 minutes in group I and 72.0+/-3.7 minutes in group II, the latter being significant (p<0.001). It was, however, 49.0+/-3.6 minutes in group III, indicating that SCh shnwed a little effect, while it was 38.1+/-1.8 minutes in group IV, being significantly shorter than those of group I and IlI (p<0.01). 3. Mean potency of atracurium expressed by the percentage change of initial twich height was 0.7+/-0.5% in group I and it was decreased to 0% in group II but it was not sinificant. In group III, it was increased to 0.9+/-0.6% but it was also no significance. In group IV, it was decreased to 0.4+/-0.2% which was not significant. 4. Presence of atracurium (0.5 mg/kg) in group IV did not have any effect on the intensity of fasciculation induced by SCh. These results indicate that succinylcholine did not attect the onset time and action potency of atracurium. However, in group of the mixed injection, SCh shortened only the duration of action of atracurium.