Cimetidine, and H2-receptor blocker, is associated with a direct inhibition of liver microsomal enzymes and a decrease in liver blood flow which results in a variety of clinically significant drug interactions, but its effects on the action of muscle relaxants have not been established. Recently, Kambam et al(1987) reported that the duraion of action of succinylcholine was prolonged 2~2.5 times by cimetidine. The effects of cimetidine, ranitidine, and famotidine, on duration of action of succinylcholine and vecuronium were studied. Sixty ASA class 1 or 2 patients scheduled for elective exploratory laprotomies were randomly divided into two groups as follows: Succinylcholine Group(n=40, 1mg/kg) control group(n=10), cimetidine group(n=10): 300mg ph hs & 1 hour before induction, ranitidine group(n=10): 150mg po hs & 1 hour before induction, famotidine group(n=10) : 40mg po hs only. Vecuronimu Group(n=20, 0.08mg/kg) control group(n=10), famotidine group(n=10): 40 mg po hs only. All patients were premedicated with hydroxyzine(1.5mg/kg) and Robinul 0.2mg IM, 1 hour before anesthesia. Anesthesia was induced with thiopental sodium 5~6mg/kg and succinylcholine 1mg/kg or vecuronium 0.08mg/kg. Tracheal intubation was performed at 75~!00% block as monitored by train-of-four stimulation of the ulnar nerve at 2Hz, 20 seconds apart(ABM, Datex). Anesthesis was maintained with 50% nitrous oxide, oxygen, and 2% enflurane. The duration of action of the muscle relaxants was measured from injection of muscle relaxant to the time of 25% recovery of first twitch height on train-of-four stimulation. The results were as follows: 1) The duration of succinylcholine was prolonged significantly by the H2-receptor blockers, 8.3+/-1.49 minutes with cimetidine, 9.8+/-1.98 minutes with ranitidine, and 10.3+/-2.48 minutes in the famotidine group as compared to 6.9+/-1.43 minutes in the control(p<0.05). 2) The duration of action of vecuronium was not affected significantly by the H2-receptor blocker, 26.5+/-4.72 minutes in the famotidine group as compared to 23.6+/-4.52 minutes in the control(p>0.05).