To evaluate possible potentiation of neuromuscular blocking effect of a combination of vecuronium and d-tubocurarine, cumulative dose-response curves were constructed to compare the potency of this combination with vecuronium and d-tubocurarine given alone. Ten patients each were given incremental injections of 80 micrograms/kg d-tubocurarine or 5 micrograms/kg vecuronium plus 40 micrograms/kg d-tubocurarine, the data for incremental administration of 10 micrograms/kg vecuronium being used from our previously published study (7). The results showed the combination of vecuronium plus d-tubocurarine to be significantly more potent (P less than 0.05) than would be expected from a simple additive effect of the individual drugs given alone. The ED95 doses of d-tubocurarine and vecuronium were 530 micrograms/kg and 57 micrograms/kg, respectively, when administered alone, but when administered together, the ED95 doses were 160 and 20 micrograms/kg, respectively.