Previous studies examined the neuromuscular effects of mivacurium in doses up to, but not exceeding, 2.5 times 95% effective dose (ED95) in children. To determine whether larger doses offer clinical advantages, we compared the onset and duration of neuromuscular block, intubating conditions, and changes in plasma histamine concentration (PHC) after mivacurium (0.2, 0.3, or 0.4 mg/kg) with those after succinylcholine (2.0 mg/kg) during propofol/N2O anesthesia in 48 children aged 3-10 yr. The evoked electromyograph (EMG) of the adductor digiti minimi after supramaximal train-of-four (TOF) stimulation was recorded. When T1 was 10% of control, laryngoscopy and intubation were performed. PHC was measured immediately before and at 2 and 5 min after administration of the relaxant. Venous blood was sampled for determination of plasma cholinesterase activity. Axillary temperature was measured. Increasing the dose of mivacurium from 0.2 to 0.3 mg/kg accelerated the onset of block (time to 90% block, 1.6 +/- 0.2 vs 1.2 +/- 0.2 min) (P < 0.001), but did not significantly prolong recovery (time to 95% recovery, 16.0 +/- 3.8 vs 18.6 +/- 3.6 min). A further increase in dose to 0.4 mg/kg produced no significant decrement in onset time, but did prolong recovery (time to 95% recovery, 23.8 +/- 5.0 min) (P < 0.001). The duration of action of mivacurium 0.3 and 0.4 mg/kg correlated inversely with plasma cholinesterase activity. PHC increased significantly after mivacurium 0.3 and 0.4 mg/kg; however, mean arterial pressure did not change significantly. We conclude that mivacurium 0.3 mg/kg provides a relatively rapid onset and short duration of neuromuscular block in healthy children. Increasing the dose to 0.4 mg/kg does not significantly accelerate the onset of neuromuscular block.