The interaction between two non-depolarizing neuromuscular blocking agents, pancuronium bromide and vecuronium bromide, has been studied at standardized levels of neuromuscular blockade and alternating the sequence of their administration, in 40 surgical patients. The drug administered first appeared invariably to play a dominant role in influencing both the dose requirements and the duration of action of the subsequent neuromuscular blocker. This resulted in reduced dose requirements and significant prolongation of action of vecuronium administered after pancuronium and increased dose requirements and shortening of neuromuscular blocking action of pancuronium given during vecuronium-induced partial neuromuscular blockade. Possible mechanisms of such interaction are discussed.