The effects of morphine upon the transmission of nociceptive messages at the spinal level have been investigated in spinal cats by studying its effects on the activities of lamina V dorsal horn interneurons. Morphine (2 mg/kg i.v.) induced a direct depressive action at the spinal level, since it strongly reduced both spontaneous and evoked activities of lamina V cells. The spontaneous firing rate and the responses elicited by natural nociceptive stimulation were decreased by 50%. The responses of these units evoked by supramaximal electrical stimulation were reduced to 67% of their initial value; in this case, the depressive effect was much more prominent on the late component of the long duration responses. The observed depressive effects are specific since they are immediately reversed by administration of opiate antagonists (nalorphine or naloxone).