The non-depolarising muscle relaxant vecuronium (0.2 mg kg-1) was administered to four dogs. At 50 per cent return of neuromuscular activity, as measured by the train-of-four technique, the depolarising muscle relaxant suxamethonium (0.3 mg kg-1) was injected intravenously. At 50 per cent return of neuromuscular activity reversal of the block was achieved with atropine and neostigmine. The duration of action of suxamethonium was reduced by the prior administration of vecuronium. In the second series of experiments the order of administration of the suxamethonium and vecuronium was reversed. Suxamethonium (0.3 mg kg-1) was administered first and at 50 per cent recovery vecuronium (0.2 mg kg-1) was given. At 50 per cent recovery of the twitch response after vecuronium administration the block was reversed with atropine and neostigmine. The previous administration of suxamethonium prolonged the duration of the vecuronium induced neuromuscular block.