The effects of intravenous diazepam (0.2 mg/kg) on the evoked electrosinogram recorded with an epidural electrode in the posterior epidural space of the lumbar enlargement and on the evoked electromyogram recorded with disc electrodes on the gastrocnemius muscle were studied following posterior tibial nerve stimulation in fourteen subjects. Following administration of diazepam, the amplitude of P_1, a reflection of afferent input through the dorsal root, was significantly depressed 3 min after intravenous administration. The amplitude of P_2 of EESG, a reflection of primary afferent depolarization in the spinal cord was significantly increased 10-30 min after administration. The amplitude of the H-reflex of the evoked electromyogram decreased significantly 3 to 30 min after administration, whereas that of the M-wave remained unchanged. These results may indicate that diazepam in the clinical doses directly affects the function of the human spinal cord.