We compared the neuromuscular and cardiovascular changes following administration of mivacurium 0.15, 0.20 and 0.25 mg kg-1, suxamethonium 1.0 mg kg-1 or atracurium 0.5 mg kg-1 i.v. in 41 (ASA physical status I or II) patients during nitrous oxide-fentanyl anaesthesia. Mean onset times for total ablation of twitch response for mivacurium 0.15, 0.20 and 0.25 mg kg-1, were 2.5, 2.4 and 2.7 min, respectively, similar to that for atracurium (2.5 min), but longer than for suxamethonium (1.1 min) (P less than 0.05). Mean times from administration of drug until twitch response recovered to 10% of control were shorter for each dose of mivacurium (15.6, 18.0 and 20.6 min, respectively) than for atracurium (40.0 min) and longer than for suxamethonium (7.7 min) (P less than 0.05). Mean infusion rate required to maintain twitch response at 5 +/- 4% control was 6.7 micrograms kg-1 min-1 for mivacurium and 6.3 micrograms kg-1 min-1 for atracurium. Following neostigmine 0.045 mg kg-1, mean times for twitch tension to recover from 10% to 90% of control were similar for mivacurium (9.7 min) and atracurium (10.5 min). Transient decreases in mean arterial pressure (greater than 20%) were observed in seven of 15 patients who received the two higher doses of mivacurium.