BACKGROUND: The dose-responses of neuromuscular blocking agents may be influenced by many factors including age and inhalation anesthetics. This study was designed to determine the dose-response relationships of a new, short-acting muscle relaxant, mivacurium during nitrous oxide-halothane or nitrous oxide-enflurane anesthesia in two age groups, infants and 1 to 6 years old preschool children. METHODS: Neuromuscular blockade was monitored by recording the accelerographic activity of the adductor pollicis muscle resulting from supramaximal stimulation at the ulnar nerve at 2 Hz for 2 seconds at 10-second intervals. To estimate dose-response relationships, 24 infants or children of two anesthetic subgroups for each age group received single bolus doses of 45~100 g/kg of mivacurium. The ED50 and ED95 were estimated from linear regression plots of log-dose vs probit of twitch depression. The lag time, onset time and maximal depression of twitch height for the selective medium dose were mesured. RESULTS: The ED50 and ED95 for the infants group were 38.2 and 53.3 g/kg during halothane anesthesia, and 29.8 and 48.6 g/kg during enflurane anesthesia, respectively. And, those for preschool children group were 49.4 and 90.7 g/kg during halothane anesthesia, and 32.3 and 81.4 g/kg during enflurane anesthesia, respectively. There was a parallelism of the dose-response curve between halothane and enflurane anesthesia in either age group. Also, there was statistically significant difference in the maximal twitch depression for the selective medium dose of mivacurium between halothane and enflurane anesthesia in either group. CONCLUSIONS: The potency of mivacurium during enflurane anesthesia is higher than that during halothane anesthesia in infants and preschool children, and during either inhalation anesthesia the dose of mivacurium is less required in infants than preschool children.