The tetanic and single twitch responses of the adductor pollicis muscle were used to study the neuromuscular effects of neostigmine in 26 patients anaesthetized with thiopentone and nitrous oxide. Neostigmine 2.5 mg i.v. given 5 min after exposure to halothane antagonized non-depolarizing neuromuscular block, whereas a second dose give 2-5 min later depressed the peak tetanic contraction and re-established tetanic fade. In the absence of halothane the second dose of neostigmine had less effect. Recovery of the single twitch was not impaired by the second dose. A single dose of neostigmine 5 mg rapidly antagonized the competitive block of the tetanic response but the subsequent slight depression of the peak contraction and the brief reappearance of fade were less than after 5 mg given in two doses of 2.5 mg. In patients who were not given neuromuscular blocking drugs, one or two injections of neostigmine 2.5 mg caused a substantial reduction in the peak tetanic contraction and severe tetanic fade which persisted for about 20 min; the single twitch was slightly potentiated. The neostigmine block of the tetanic response could be antagonized by gallamine and potentiated by suxamethonium. These findings indicate that neostigmine in clinical doses can produce an acetylcholine-induced block which be a potential hazard in anaesthetic practice.