Evoked potentials and EEG are used to monitor the central nervous system and the depth of anesthesia in anesthetized patients. In this study, we examined EEG, VEP, SEP and ABR at various concentrations of isoflurane or sevoflurane, and evaluated the influence of volatile agents and their usefulness for the monitoring of the depth of anesthesia. With increasing concentrations of isoflurane and sevoflurane, AE (frequency which account for 80% of total voltage) showed dose-related reduction and EEG showed a trend toward a slower wave and higher amplitude. With increasing concentrations of isoflurane and sevoflurane, P100 of VEP showed a significant dose-related reduction in its amplitude and increase in its latency. So we could not record P100 at the level of the anesthesia of 1.0 MAC with 66% N2O. N20 of SEP can be easily recorded at any depth of anesthesia. And the trend showed consistent dose-related changes in amplitude and latency. With increasing concentrations of isoflurane and sevoflurane, wave III and V of ABR increased significantly in latency, but the changes were very small. In conclusion, the effect of isoflurane and that of sevoflurane on evoked potentials and EEG are similar with each other and with other volatile agents. SEP is the most consistent and reliable factor to monitor the depth of anesthesia neurophysiologically.