Many methods are available for monitoring spinal cord conductian during operation. It is essential for the anesthesiologist to the know potential interactions between the drugs used during the operation and the neurophysiologic response. The purpose of the present study was to experimentally examine the effects of ketamine and fentanyl used in the operating room. Twenty-three patients, 23 to 68 years of age, ASA I-II, free of neurologic disease and scheduled for elective operations were randomly selected for the study. Averaged somatosensory evoked potentials to stimulation of the posterior tibial nerve at the ankle were recorded. After baseline recording of the somatosensory evoked potential, ketamine (1.5 mg/kg) or fentanyl (3μ/kg) was administered by bolus injection in each patient. Additional somatosensory evoked potential after the drug injection was recorded. ECG, blood pressure and heart rate were monitored throughout the measuring procedure. 1) The mean values of latency (Pl, Nl, P2) and amplitude (Pl-N1, Nl-P2) were 39.58, 48.36 and 60. 74 msec and 7.88 and 10.30μV respectively. Those values were changed to 38.54, 46.67 and 60.50 msec and 6.55 and 9.85μV in patients in the ketamine injected group. 2) The mean values of latency and amplitude were 37.90, 45.93 and 61.01 msec and 7.35 and 7.21μV respectively in the baseline state. Each value changed to 38.10, 47.06 and 61.88 msec and 6.67 and 8.49μV respectively after fentanyl injection. There was no statistical difference in latencies and amplitudes between the pre-injection state and post-injection state in both the ketamine and fentanyl groups. Therefore, we suggested that the use of ketamine or fentanyl could be administered successfully during intraoperative somatosensory evoked potential monitoring without much influence on tibial nerve stimulation.