The priming principle of non-depolarizing muscle relaxants was treated in this study. The subjects were 48 patients. We administered divided doses (p) and single dose (s) using pancuronium (P) and vecuronium (V), and compared the 4 groups (Pp, Ps, Vp, Vs). Pp and Vp groups received intravenous injection of 0.02 mg.kg-1 first, and 0.08 mg.kg-1 after 5 minutes. Ps and Vs groups received intravenous injection of 0.1 mg.kg-1. Relaxograph was used for monitoring muscle relaxation. First-twitch (T1) and train-of-four ratio (TR) were recorded, and the time intervals required to decrease T1 to 25% (T1-25) and 5% (T1-5) were determined. T1-25 (sec) was 126.2, 153.1, 82.7, and 132.7 in Pp, Ps, Vp, and Vs group, respectively; and T1-5 (sec) was 192.8, 229.8, 112.7, and 165.2 in Pp, Ps, Vp, and Vs group, respectively. As these values suggest, there were no significant differences between Pp and Ps group, while significant differences were noted between Vp and Vs group. The following conclusions were obtained. Clinical usefulness of the priming principle was not shown with pancuronium, but was noted with vecuronium. However, the action of vecuronium appeared rapidly after single dose, and some patients complained of dyspnea during priming. Consequently, the priming principle was not considered to be clinically beneficial.