BACKGROUND: Sevoflurane is a new inhalation anesthetic, which shows stable hemodynamic features, and does not have a pungent odor, so that it can be used as an inhalational induction agent. But mask ventilation can make patients anxious, and can stimulate the sympathetic nervous system. Therefore, we evaluated the effect of midazolam administered right before inhalation induction with sevoflurane on the autonomic nervous system using power spectral analysis. METHODS: We studied 28 patients undergoing elective surgery. They were assigned to one of two groups to receive either sevoflurane with midazloam (M group) or sevoflurane alone (S group). Anesthesia was induced with 6% sevoflurane and 0.03 mg/kg midazolam or 6% sevoflurane alone. Midazolam was administered immediately before induction with sevoflurane. An electrocardiogram was applied to all patients and connected to a computer, for power spectral analysis. Power values were recorded on arrival, pre-intubation, post-intubation and pre-incision, and compared between two groups. RESULTS: Power values at low frequency were lower, and the time to loss of eye lash reflex and induction were shorter in the M group than in the S group. CONCLUSIONS: The administration of midazolam before induction with sevoflurane reduced the activity of the sympathetic nervous system versus induction with sevoflurane alone.