In various animal species anaesthetized with a-chloralose (cats and pigs) or pentobarbitone (Beagle dogs and Rhesus monkeys), rocuronium has been shown to be a readily reversible, non-depolarizing neuromuscular blocking agent with a similar duration of action as vecuronium but a 6-10 fold lower potency. The outstanding features of its action is rapidity of onset. It is not expected to have any marked cardiovascular or autonomic side-effects when used in the neuromuscular blocking dose range. There is no evidence of any selective pre-junctional effect and there is no clinically relevant inhibition of acetycholinesterase. Screening in rats has not demonstrated any oestrogenic, androgenic, anabolic, glucocorticoid-like or gonad-inhibiting properties, although there was a slight increase in pituitary weight in male rats.