Simultaneous recordings of responses of substantia gelatinosa and deep dorsal horn neurones to thermal noxious cutaneous stimulation were made in spinalized cats anaesthetized with urethane/chloralose. Morphine, whether applied iontophoretically in the substantia gelatinosa (50-200 nA) or injected intravenously (1.0-1.5 mg/kg), enhanced the responses of the substantia gelatinosa cells while depressing those of deep cells. Met-enkephalin-amide (50-200 nA) also had similar reciprocal actions. Naloxone counteracted these effects of the agonists. The results support our previous proposal that the opiates facilitate the activity of a substantia gelatinosa system that controls the responses of deep dorsal horn neurones to pain.