Succinylcholine is used most widely for tracheal intubation during induction for general anesthesia. However adverse effects following the administration of succinylcholine which include an increase of serum potassium, creatine phosphokinase, abdominal pressure and the presence of fasciculation are well known to anesthesiologists. Many investigators have studied the influence of pretreatment with d-tuhocurarine and diazepam to prevent the adverse effects following administration of succinylcholine. We studied the effects of serum potassium and creatine phophokinase with midazolam(0.25 mg/kg) the most newly introduced benzodiazepine derivatives and thiopental sodium(4-5mg/kg) the most widely used induction agent for anesthesia in 33 adult healthy surgical patients. We also observed the onset of loss of eyelash reflex and fasciculation, the degree of fasciculation and the adequacy of relaxation for intubation comparing the two agents. The results are as follows: 1) There were no significant increases in serum potassium and creatine phosphokinase levels before succinylcholine and after 10 minutes administration in both agents groups. 2) Loss of eyelash reflex occurred immediately with thiopental and at 63±43 seconds with midazolam one minute after administration. 3) Fasciculation appeared at 14.4±4.9 seconds with thiopental and at 21.3±11.9 seconds with midazolam and continued 66.3±20.1 seconds with thiopental and 43.5±l8.5 seconds with midazolam. 4) The adequacies of relaxation for intubation were good enough in both agents groups but midazolam seems to be better. Midazolam seems to be a good induction agent for general anesthesia and prevents an increase of serum potassium level after administration of succinylcholine without pretreatment of non-depolarizing muscle relaxants or diazepam.