BACKGROUND: There were many reports about the effect of succinylcholine on the action of nondepolarizing muscle relaxants. The results are inconsistent depend on the nondepolarizing muscles relaxants used, time when nondepolarizing blockers administered and methods of experiments etc. We investigated the effect of succinylcholine on the neuromuscular blockade induced with mivacurium, a new short acting nondepolarizing muscle relaxant, when mivacurium was administered during early and late recovery from succinylcholine block and when different dose of succinylcholine were used. METHODS: 30 rabbits were divided into 5 groups including control group. Control group was administered mivacurium only. In other 2 groups, succinylcholine (3xED95) was administered, and mivacurium was given at 5% and 100% recovery from succinylcholine. In the other two groups, succinylcholine (6xED95) was administered, and mivacurium was given at 5% and 100% recovery from succinylcholine. We investigated onset time, duration of relaxation, and recovery index of mivacurium induced neuromuscular block. RESULTS: Onset time was shortened in all groups compare to control group. Duration and recovery index were not changed significantly at 5% and 100% recovery of succinylcholine (3xED95) administered group, but prolonged significantly (p<0.05) in succinylcholine (6xED95) administered groups compare to control group. CONCLUSION: Mivacurium induced block were more prolonged at 100% recovery of succinylcholine (3xED95) induced block and these effect were more potentiated by the increasing the dose of succinlycholine (6xED95) administered group.