OBJECTIVE The effects of volatile anesthetics on change in the bispectral index (BIS) due to tracheal intubation are unclear. We investigated hemodynamic and BIS responses to intubation during isoflurane or sevoflurane anesthesia. METHODS After obtaining Institutional Review Board approval and informed consent, we randomly allocated 40 patients of American Society of Anesthesiologists (ASA) physical status I to receive either isoflurane (ISO group; n = 20) or sevoflurane (SEV group; n = 20). The patients were anesthetized with thiamylal and were ventilated with 100% oxygen, using a mask. The inspired concentrations of isoflurane and sevoflurane were gradually increased and maintained at end-tidal anesthetic concentrations of 2 minimum alveolar concentration (MAC) during the study period. Tracheal intubation was performed 15 min after the end-tidal anesthetic concentrations had reached 2 MAC. Mean arterial pressure (MAP), heart rate (HR), and BIS were recorded before induction, at the loss of consciousness, before laryngoscopy, and at 1, 3, and 5 min after intubation. RESULTS Anesthesia with 2 MAC volatile anesthetics increased HR in the ISO group, and decreased MAP in the SEV group. The BIS value decreased from 95 +/- 3 and 96 +/- 2 before thiamylal to 39 +/- 9 and 38 +/- 10 before intubation in the ISO and SEV groups, respectively. MAP and HR were significantly increased in both groups 1 and 3 min after intubation, but BIS remained unchanged. CONCLUSIONS Anesthesia with 2 MAC of isoflurane and sevoflurane was effective to suppress the change in BIS due to intubation but was not sufficient to prevent changes in hemodynamic responses.