BACKGROUND: Because rocuronium is a new steroid nondepolarizing muscle relaxant with a faster onset time than others, it is a very useful drug that can be substituted for succinylcholine. The aim of the present study was to evaluate the onset time and hemodynamic effects in children after application of approximately equipotent doses of atracurium, vecuronium, and rocuronium. METHODS: ixty children (ASA physical status 1) were randomly assigned to receive a single dose of either atracurium (0.5 mg/kg), vecuronium (0.1 mg/kg) or rocuronium (0.6 mg/kg). All children were premedicated with midazolam (0.05 mg/kg) and glycopyrrolate (0.004 mg/kg) an hour before induction. After induction of anesthesia with thiopental sodium (5 mg/kg), the lungs were ventilated with 50%nitrous oxide in oxygen and 2 vol% sevoflurane via a face mask. The hemodynamic changes were monitored every minutes for 5 minutes. In addition, after administration of the muscle relaxant the evoked electromyogram of the adductor pollicis was measured after train-of-four stimulation (TOF) at 2 Hz every 10 seconds during 5 minutes. The onset time was defined as the first response of the TOF (T1) was disappeared. We also investigated intubating conditions. RESULTS: There were no differences between groups in respect of age, weight, height, and gender. The onset time of rocuronium was faster (99.84+/-20.46) than vecuronium (149.54+/-19.45) and atracurium (176.36+/-23.01). There were no differences between groups in the hemodynamic changes and intubating conditions. The patient's movement on injection was frequently observed in vecuronium and rocuronium. CONCLUSIONS: The onset time of rocuronium was the fastest among the three muscle relaxants. So, rocuronium was a very useful drug for rapid sequence intubation in children, although the onset time of rocuronium is a little slower than succinylcholine.