The neuromuscular effect of neostigmine, 1.25 mg/70 kg, was assessed in 40 adult patients 10 min after cessation of a succinylcholine infusion. The patients had received a thiopental-nitrous oxide anesthetic supplemented by halothane or fentanyl during which they were given at least 5 mg/kg succinylcholine over more than 90 min. Train-of-four monitoring was used. Neostigmine accelerated recovery of neuromuscular function in all patients. The degree of recovery was directly related to the train-of-four ratio, and the results in patients who had received halothane were no different from those who had received fentanyl. The findings are compatible with the hypothesis that phase I block depends upon the presence of circulating succinylcholine and decreases as the latter is cleared, whereas phase II block decreases more slowly. Thus succinylcholine block can be antagonized by neostigmine if enough time is allowed for phase I block to disappear and for a pure phase II block to be present.