Somatosensory evoked potentials (SSEP) are used increasingly to monitor the integrity of neural pathways in anesthetized patients. To evaluate the influence of epidural anesthesia on the central nervous system, we studied the effects of lumbar or thoracic epidural anesthesia with lidocaine on the median nerve SSEP in 9 patients. The peak latencies (N1, P2, N2) and amplitudes (N1-P2, P2-N2) of the SSEP response over the sensory cortex were recorded before and 15 min after epidural anesthesia. The peak latencies of control and post epidural anesthesia of N1, P1, N2 were 19.2 +/- 1.7 msec, 19.6 +/- 1.6 msec (N1), 24.7 +/- 2.3 msec, 25.7 +/- 2.0 msec (P2), 32.8 +/- 2.8 msec and 34.6 +/- 2.5 msec (N2), respectively. The amplitude of control and post epidural anesthesia of N1-P2, P2-N2 were 4.5 +/- 2.9 microV, 5.9 +/- 6.6 microV (N1-P2), 4.4 +/- 3.2 microV and 5.6 +/- 5.2 microV (P2-N2), respectively. Peak latencies of all components (L1, P2, N2) increased after epidural anesthesia compared with control values. Amplitude of N1-P2 increased significantly following epidural anesthesia compared with control values. The data obtained in this study were contrary to the previous concept that anesthetic agents generally increase the latency of SSEP and decrease their amplitude.