The effects of inhalation anesthetics (enflurane, halothane, nitrous oxide) and hyperventilation on brainstem auditory evoked responses (BAER) were studied in 52 patients underwent lower abdominal surgeries with spinal continuous peridural anesthesia, that was able to exclude noxious stimuli of the surgical procedures. Under normothermic, normocapnic and normotensive status, with increasing end-tidal concentration of enflurane and halothane, the interpeak latencies (IPL) I-II, III-V and I-V increased with a linear dose-related manner. While, with nitrous oxide (66%), IPL I-III increased, but IPL III-V didn't change. This results showed that enflurane and halothane influenced the auditory pathway from the acoustic nerve to the thalamus with a linear dose-related manner. However, nitrous oxide was different from halogenized agents, such as enflurane and halothane, in the effects on the brainstem. Under hyperventilation (PaCO2 30-25mmHg) with 1.0% enflurane, IPL I-V increased significantly in comparison with normoventilation (PaCO2 45-35mmHg). High concentratian of enflurane with hyperventilation should be given carefully, bacause BAER latency delay was considered to be related with seizure activity of enflurane.