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We determined the dose-response relationships of mivacurium (BW B1090U) in children (2-10 years) during nitrous oxide-halothane anesthesia (0.8% end-tidal) and during nitrous oxide-narcotic anesthesia. Neuromuscular blockade was monitored by recording the electromyographic 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, for each anesthetic background four subgroups of nine patients received single bolus doses of 20-120 μg/kg mivacurium. The ED50 and ED95 (estimated from linear regression plots of log-dose vs. probit of effect) were 52 μg/kg and 89 μg/kg during halothane anesthesia and 62 μg/kg and 103 μg/kg during narcotic anesthesia. Nine additional patients in each anesthetic group received 250 μg/kg mivacurium. Three of the 18 patients given 250 μg/kg mivacurium developed cutaneous flushing; in one of these mean arterial pressure decreased 32% for less than 1 minute; no significant changes in heart rate occurred. With the increase in mivacurium dose from 120 μg/kg to 250 μg/kg the times to onset of 90% and maximum neuromuscular block decreased by 0.5 to 1 minute, and the times to recovery of neuromuscular transmission to 5% (T5) or 25% (T25) increased by 2-4 minutes. The recovery index (T25-75) in patients anesthetized with halothane was 4.3 ± 1.5 minute (mean ± SD); the time to complete recovery (T4:1 ≥ 0.75) was 19.8 ± 7.4 minutes.