Subparalyzing dose of nodepolarizing relaxants prior to injection of succinylcholine has been used to prevent various adverse effects induced after succinylcholine. For investigating interactions between succinylcholine and small doses of four non-depolar-izing agents, the 112 subjects that were ASA class 1-2 and no existing neuromuscular conduction system disorder were divided into 5 groups that were control group(only succinylcholine 1 mg/kg) and pretreated group d-tubocurarine 0.5 mg/kg, atracurium 0.08 mg/kg, vecuronium 0.01 mg/kg and pancuronium 0.01 mg/kg. In each group, the plasma concentration of K+ and PChE before and after use of succinylcholine, fasciculation, onset and recovery time of succinylcholine block and intubating conditon were observed. The results are as follows; In the pretreated group, there were no significant changes of plasma concentration of K+ and plasma cholinesterase(Table 3) but diminished the incidence of fascieulation, delayed the onset time and shorted the recovery time of succinylcholine block(Table 4), and worse in intubating condition(Table 5) except pancuronium treated group. It was concluded that these seem to make worse condition of intubation, while small doses of nondepolarizing muscle relaxants except pancuronium antagonize depolarizing muscle relaxant.