Eighty-one pediatric patients, ranging from neonates to adolescents, were studied during fentanyl-N2O-O2 anesthesia to determine for each of them the vecuronium infusion required to maintain 90-95% neuromuscular block (NMB). Electromyographic monitoring with train-of-four stimuli was used. The steady infusion rate was 62 +/- 15 (SD) micrograms.kg-1.hr-1 in neonates and infants. This rate was 40% of that required by children 3 to 10 years old (154 +/- 49 micrograms.kg-1.hr-1; P less than 0.05). In adolescents the vecuronium requirement was less than in children and was comparable to that reported in adults in other studies (89 +/- 13 micrograms.kg-1.hr-1). Despite considerable individual variation, the infusion rate could be reliably estimated on the basis of duration of greater than 90% NMB maintained by small doses of vecuronium given after intubation. Also, a close correlation existed between the duration of greater than 90% NMB maintained by 100 micrograms/kg of vecuronium and the individual infusion rate (r2 = 0.76).