Mivacurium has been little studied in infants and children without a volatile anaesthetic agent. We analysed onset time and maximal neuromuscular response after mivacurium 0.1 mg/kg, and the infusion requirement of mivacurium to maintain a 50, 90, or 95% neuromuscular block in 76 infants and children under N2O-O2-alfentanil anaesthesia. Furthermore, we assessed the time course of potentiation of 1 MAC end-tidal halothane or isoflurane on the infusion requirement of mivacurium. Neuromuscular response was recorded by adductor pollicis electromyogram. The onset time of mivacurium was shorter in infants than in children (2.1 +/- 0.6 and 3.2 +/- 0.9 min (mean +/- SD); P = 0.0001). The dose potency of mivacurium did not depend on the age of a paediatric patient. The estimated ED95 of mivacurium was 136 +/- 46 micrograms/kg. The mivacurium requirement to maintain a 50, 90, or 95% neuromuscular block averaged 340, 730, and 900 micrograms/kg/h, respectively. Halothane and isoflurane decreased this hourly requirement by 35 and 70%, respectively. The decrease in the mivacurium infusion requirment was fastest in the youngest children. In conclusion, mivacurium is easy to administer as bolus doses or continuous infusion in paediatric patients because its potency is similar in all patients from 1 month to 15 years of age. Halothane and isoflurane produce their maximal potentiation of neuromuscular block only after 30-60 min of administration. This potentiation is similar in magnitude in all patients, but takes place fastest in the youngest children.