The neuromuscular blocking effects of mivacurium during sevoflurane or halothane anaesthesia was studied in 38 paediatric patients aged 1-12 yr. All received premedication with midazolam, 0.5 mg.kg-1 po and an inhalational induction with up to 3 MAC of either agent in 70% N2O and O2. The ulnar nerve was stimulated at the wrist by a train-of-four stimulus every ten seconds and the force of adduction of the thumb recorded with a Myotrace force transducer. Anaesthesia was maintained with a one MAC end-tidal equivalent of either volatile agent for five minutes before patients received mivacurium (0.2 mg.kg-1) iv. The onset of maximal blockade occurred in 2.4 +/- 1.26 (mean +/- SD) min with halothane and 1.8 +/- 0.54 min with sevoflurane (NS). Four patients failed to achieve 100% block (3 halothane, 1 sevoflurane). The times from injection to 5, 75, and 95% recovery during sevoflurane (9.8 +/- 2.6, 19.5 +/- 4.4, and 24.2 +/- 4.8 min) were greater than during halothane anaesthesia (7.2 +/- 2.2, 15.0 +/- 4.0, 19.2 +/- 4.9 min, respectively (P < 0.005). All patients demonstrated complete spontaneous recovery of neuromuscular function (T1 > 95%, T4/T1 > 75%) during the surgery which lasted 24-63 min. All patients showed clinical signs of full recovery of neuromuscular blockade (i.e., headlift, gag, or cough). Pharmacological reversal was not required. It is concluded that following a single intubating dose of mivacurium, the time to maximum relaxation was not different during halothane and sevoflurane anaesthesia; recovery times to 5, 75 and 95% twitch height were longer during sevoflurane anaesthesia and neuromuscular reversal was not necessary.