Atracurium is a potent competitive neuromuscular blocking agent in anesthetized man with no cardiovascular effects at doses required for paralysis. Endotracheal intubation can be accomplished after i.v. doses of 0.6 and 0.3 mg/kg, within 1 and 2 min, respectively. Paralysis is readily antagonized by neostigmine and is enhanced by halothane. The consistent response in terms of block and recovery which emerged when the drug was given as increments of 0.05 or 0.1 mg/kg indicates the absence of cumulative effects. The course of action of atracurium was appreciably shorter than that of other recognized competitive blocking agents. Doses of 0.3-0.6 mg/kg i.v. provided adequate relaxation during surgical intervention for 15-45 min; spontaneous recovery without the use of neostigmine was observed in some patients. In addition to the non-enzymic decomposition by Hofmann elimination, atracurium may undergo an enzymic ester hydrolysis but, unlike suxamethonium, it may not be destroyed by pseudocholinesterase.