BACKGROUND: It is important to consider the sequential administration of long- and short-acting non-depolarizing muscle relaxants. The purpose of this study is to evaluate the interaction between non-depolarizing muscle relaxants under isolated forearm test. METHODS: Ethics committee approval for all the volunteers (n=12) experiments was obtained. Pancuronium, 0.3 mg in 20 mL saline, was injected into one forearm; and vecuronium, 0.3 mg in 20 mL saline, was injected simultaneously into the other forearm in 6 volunteers. Three minutes later both tourniquets were released. Following spontaneous recovery in each forearm to 50% of the control twitch, the tourniquet was reinflated and the dose of drug of the same dilution used in the contralateral forearm was then injected (i. e., vecuronium following pancuronium in one forearm and pancuronium following vecuronium in the other forearm). Tourniquet was again released after 3 minutes and spontaneous recovery was allowed to occur. Interactions between pancuronium (0.3 mg) and rocuronium (2.0 mg) in 6 volunteers were also evaluated in a same method. RESULTS: Prior administration of pancuronium significantly prolonged the recovery rate from subsequent rocuronium (23.9 vs. 32.1 min, p<0.05), and rocuronium significantly shortened the recovery rate of pancuronium (32.4 vs. 25.9 min, p<0.05). There was simple additive interaction between pancuronium and vecuronium. CONCLUSION: The possible mechanism of these phenomena may be due to interaction of drug with acetylcholine receptors in the neuromuscular junction rather than due to residual plasma drug concentration.