It has been already reported that diazepam pretreatment attenuated succinylcholine(Sch)-induced myalgia, fasciculation and potassium elevation. The effect of midazolam, a benzodiazepine derivative like diazepam, on Sch has not been investigated. So Midazolam, d-Tubocurarine and diazepam pretreatment were studied to determine the effect on subsequently administered Sch in 36 female patients. The patients were divided into 4 groups. Group 1 received no pretreatment, only Sch 1mg/kg(control). While Group 2 received d-Tubocurarine 0.05mg/kg, Group 3 received diazepam 0.05 mg/kg, and Group 4 received midazolam 0.025 mg/kg. After the pretreatment 4 minutes before Sch administration, the time to abolition and recovery of twitch height, the degree of fasciculation, the adequacy of relaxation for intubation, the changes in serum potassium and creatine pliosphokinase(CPK) were measured. The results are as follows: I) Group 2 showed increased time to abolition of twitch and decreased time to recovery of twitch. Group 3 showed increased time to recovery of twitch. 2) The incidence of fasciculation was decreased in Groups 2, 3 and 4. 3) Serum potassium was increased in Groups 1 and 2 but not in Groups 3 and 4. 4) Serum creatine phosphokinase was in- creased in all groups at post-Sch. 24 hours. It is concluded that diazepam and midazolam pretreatment attenuate Sch-inducedifasciculation and potassium increase.