Recovery from neuromuscular block produced by vecuronium was studied in 50 patients using electromyography and the train-of-four technique. Twenty patients received neostigmine 2.5 mg, 10 when the initial response of the train-of-four was 50% of control and 10 when it was 10%. Neostigmine 5 mg was investigated in a similar manner and in 10 patients spontaneous recovery was studied. In all patients the time to 70% recovery of the initial response and of the train-of-four ratio was followed. Neostigmine significantly reduced the time to 70% recovery of both ratios with both degrees of block, but neostigmine 5.0 mg did not give a substantially more rapid recovery than 2.5 mg. No evidence of a neostigmine-induced block was encountered. neostigmine 2.5 mg was rapidly effective in antagonizing vecuronium-induced block, even when initial recovery was only slight: there was no advantage in using neostigmine 5.0 mg.