The depressant effects of morphine on the evoked electrospinogram (EESG), evoked electromyogram (EEMG) and nerve action potential (NAP) were studied in surgical patients. The EESG was recorded with an epidural electrode in the posterior epidural space in the lumbar enlargement. A clinical dose of morphine (1 mg/kg) depressed the amplitude of all components (P1, N1, P2) of the EESG, elicited by both weak and strong stimuli to the tibial nerve. The depressant effects of morphine were most pronounced on the P2 component of EESG, which is believed to correspond to primary afferent depolarization. The H-relfex was augmented by the drug, while the M-wave and the NAP were unaffected. The changes induced by morphine in both the EESG and the H-reflex were reversed completely, partially or even potentiated by naloxone (0.1 mg/kg). These depressant effects of morphine on the EESG were minimized under nitrous oxide (75%) anaesthesia, indicating the existence of a degree of interaction between these two drugs. Thus, morphine in a clinical anaesthetic dose, affects both spinal cord function and afferent volleys along the roots in normal man.