In this study, combined neuromuscular blocking effects of ORG9426 with other non-depolarizing neuromuscular blocking agents were investigated. About 20% steady state neuromuscular block was established by a continuous infusion of one of 6 neuromuscular blocking agents (ORG9426, vecuronium, pancuronium, pipecuronium, d-tubocurarine and metocurine). Then 1/7 of the ED50 of ORG9426 or one of other neuromuscular blocking agents was administered in a single injection, and the increase in the neuromuscular block was observed. The combined neuromuscular blocking effect of ORG9426 and d-tubocurarine or ORG9426 and metocurine was significantly ( P < 0.05) greater than that of each corresponding control (the combination of same neuromuscular blocking agent). The effect of d-tubocurarine was also potentiated by vecuronium, pancuronium and pipecuronium. These potentiations were not observed between ORG9426 and pancuronium, pipecuronium or vecuronium. Possible mechanisms of these synergistic interactions were discussed.