Naloxone (200 micrograms/kg, i.v.) reduced the noxious thermal stimuli-evoked responses of 16/25 nociceptive neurons in the superficial laminae whereas it enhanced the responses of 6/10 nociceptive neurons in the deeper dorsal horn. However, a different picture emerged when selectivity of neuronal responsivity (nocireceptive or multireceptive) was considered. In the superficial dorsal horn, naloxone reduced the responses of the majority of (15/18) selectively nocireceptive neurons. The reduction in responses became apparent within 60 sec following naloxone administration and returned to control level within 48 min. In contrast, the responses of the majority of multireceptive neurons in the superficial (6/7), or the deeper (6/10) dorsal horn, were enhanced. The excitatory action in the superficial dorsal horn persisted for only 6-15 min, whereas it persisted for 40-70 min in the deeper dorsal horn. The firing of the majority of cold-receptive neurons (6/8) in the superficial dorsal horn was not altered. These effects were stereoselective since (+)-naloxone, the inactive isomer of naloxone, did not affect the responses of 14/16 nociceptive neurons. It is concluded that naloxone differentially, and selectively, affects the firing of nociceptive neurons in the superficial versus the deeper dorsal horn, and the firing of selectively nocireceptive versus multireceptive neurons. The relevance of these findings to the behavioral effects of naloxone, hyperalgesia and analgesia, is discussed.