BACKGROUND: In this study, we assessed the 50% effective concentration (EC50) of sevoflurane for reducing a rocuronium-induced reaction, based on the Dixon's up-and-down method. We also assessed the 50 and 95% effective end-tidal concentration of sevoflurane (ETsev), based on the probit regression curve of the probability of nonwithdrawal reaction. METHODS: We conducted a prospective, double-blind study in 23 males and 24 females. After using 2.5% thiopental sodium (4 mg/kg), anesthesia was induced in the patients. The patients then inhaled sevoflurane with 5 vol% in 6 L/min of oxygen. When the target ETsev was achieved, a nurse injected the intubating dose of rocuronium (0.6 mg/kg) for 5-10 s under the free flow of intravenous fluid. After the nurse evaluated the response, the nurse recorded the maximum heart rate during 30 s and the mean arterial pressure after rocuronium injection. RESULTS: Based on Dixon's up-and-down method, the EC50 of sevoflurane was 2.5 alpha 0.5 vol% in males and 2.5 alpha 0.3 vol% in females. The probit regression curve of the probability of nonwithdrawal reaction showed that in males the 50% effective ETsev was 2.4 vol% (95% confidence interval [CI], 1.5-3.1 vol%) and the 95% effective ETsev was 3.5 vol% (95% CI, 2.9-11.0 vol%); in females, the 50% effective ETsev was 2.4 vol% (95% CI, 2.1-2.7 vol%) and the 95% effective ETsev was 3.0 vol% (95% CI, 2.7-4.5 vol%). CONCLUSIONS: The inhalation of sevoflurane during the induction period may provide a simple and reliable means of reducing rocuronium-induced reactions without adverse hemodynamic changes. There was no significant difference between males and females.