BACKGROUND: Magnesium sulfate(MgSO4) has long been used in the treatment of pre-eclampsia and hypertension. Mivacurium was found to undergo hydrolysis by plasma cholinesterase at 70-88% of the rate of succinylcholine. This study has been conducted to evaluate the time course of mivacurium-induced neuromuscular block according to the pretreatment doses of MgSO4 in rabbits. METHODS: This study was done using a common peroneal nerve-anterior tibialis muscle preparation in sixty adult rabbits. The onset and recovery times of mivacurium(0.16 mg/kg) were evaluated after the infusion of a placebo in group 1, MgSO4 30 mg/kg in group 2, MgSO4 50 mg/kg in group 3 and MgSO4 70 mg/kg in group 4, respectively(n=15 each). RESULTS: The mean serum concentration of magnesium was 2.4 mEq/L in group 1, 3.3 mEq/L in group 2, 4.3 mEq/L in group 3 and 5.5 mEq/L in group 4, respectively. The mean time from injection of mivacurium to 95% recovery of twitch amplitude was 32.7 min. in group 1, 32.9 min. in group 2, 30.4 min. in group 3 and 33.0 min. in group 4, respectively. CONCLUSIONS: The results of the present study suggested that the neuromuscular blocking actions of mivacurium are not affected according to the pretreatment doses of MgSO4.