The effects of intravenous diazepam (0.2 mg/kg) on the evoked electrospinogram 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 14 subjects. Following administration of diazepam, the amplitude of P1, and reflection of afferent input through the dorsal root, was significantly depressed 3 minutes after administration of diazepam. The amplitude of P2 of electrospinogram, a reflection of primary afferent depolarization in the spinal cord was significantly increased 10 to 30 minutes after injection. The amplitude of the H-reflex of the evoked electromyogram decreased significantly 3 to 30 minutes after injection, whereas that of the M-wave remained unchanged. These results suggest that diazepam in the clinical doses may directly affect function of the human spinal cord.