The neuromuscular effects of intravenous rocuronium bromide, 0.6 mg.kg-1 or 0.9 mg.kg-1, were studied in four groups of 10 patients during anaesthesia with or without halothane (0.5-0.75% inspired concentration). Neuromuscular block was monitored using mechanomyography and train-of-four stimulation. The mean times to onset of complete neuromuscular block were 58 and 59 s using the 0.6 mg.kg-1 dose in patients anaesthetised with fentanyl and halothane respectively. The times of 34 min and 33 min for 25% recovery of T1 (first response in the train of four), 54 min and 52 min for 90% recovery of T1, 55 min and 60 min for a train of four ratio of 0.7, and 13 and 13 min respectively for the recovery index (25-75% recovery of T1) were not significantly different in these groups. Complete block with the 0.9 mg.kg-1 dose occurred in 47 s and 44 s respectively in the fentanyl and halothane groups. T1 recovered to 25% in 51 min and 58 min, and to 90% in 77 min and 86 min respectively in the two groups. The recovery indices and the times to spontaneous recovery of the train of four ratio to 0.7 were 17 min and 19 min, and 83 min and 93 min respectively. All the parameters were significantly different between the 0.6 mg.kg-1 and 0.9 mg.kg-1 doses. Halothane in the concentrations used did not influence the neuromuscular effects. It is concluded that rocuronium is a rapidly acting non-depolarising muscle relaxant with a duration of action similar to that of vecuronium and may be a useful alternative to suxamethonium for rapid tracheal intubation.