The aim of this study was to find out whether morphine locally applied into the different subnuclei of the spinal trigeminal nucleus could impair the behav-ioral response evoked by a tonic nociceptive stimulus. Microinjections of morphine were performed unilaterally in rats through a chronically implanted tube. The duration of the formalin-induced biphasic rubbing activity was used as a measure of nociception. Morphine , 0, 2, 7, or 14 mg, in 0.2 ml of saline were microin-jected 10 min before a subcutaneous injection of forma-lin (1.5%) in the ipsilateral upper lip. Morphine microinjections into subnucleus oralis (Sp5O) and sub-nucleus caudalis (Sp5C) induced a significant decrease in rubbing duration of either the early or late phase of the response or both. The early response to formalin was depressed only after morphine injection into Sp5O. The late response was depressed by microinjections into both Sp5O and Sp5C. No significant effect was observed in subnucleus interpolaris (Sp5I). These results are further evidence for the role played by Sp5C in orofacial nociception. They also give support to the involvement of the Sp5O in perioral nociceptive mechanisms of short duration. r