BACKGROUND: Mivacurium has a characteristics of rapid onset and the shortest duration of non- depolarizing neuromuscular relaxants and the onset of action could be accelerate more rapidly by using priming principle. The purpose of this study was to compare the onset time of mivacurium by priming principle with succinylcholine during rapid endotracheal intubation. METHODS: 36 patients were randomly divided into 3 groups: mivacurium group by priming principle (Group 1), mivacurium group by bolus injection (Group 2) and succinylcholine group (Group 3). In Group 1, subparalyzing dose of 0.02 mg/kg was administered 2 minutes before principle dose of 0.25 mg/kg was given. Onset time and intubating conditions were observed when twitch tension was reduced by 25% block in each group. RESULTS: The onset of Group 1 (75 sec) was significantly faster than that of Group 2 (90 sec) (p<0.05) but was significantly slower than that of Group 3 (37.5 sec) (p<0.05). Intubating conditions were excellent in all groups. CONCLUSIONS: The attempts of priming principle with mivacurium could accelerate the onset of action of mivacurium compared with that of bolus injection but their onsets were shorter than those produced by succinylcholine.