Intracerebral administration of morphine into either nucleus reticularis paragigantocellularis (NRPG) or nucleus raphe magnus (NRM) of rats produced analgesia, as measured by the tail flick test. NRPG was more sensitive to morphine and the effect was dose dependent. The narcotic antagonist naloxone blocked these analgesic effects of morphine. The effect of intracerebral injection of naloxone on the analgesia produced by systemically administered morphine was examined. Morphine was administered subcutaneously (2.86 mg/kg) and naloxone was microinjected 35 min later. Microinjection of 5 micrograms of naloxone into NRM was found to be more effective in reversing in analgesia produced by morphine than naloxone microinjected into more lateral sites, including NRPG. Lesions of NRPG did not attenuate the analgesia produced by systemically administered morphine, whereas lesions of NRM did attenuate this analgesia. The analgesia produced by morphine administered into NRPG was blocked by lesions of NRM. Cinanserin, a serotonergic blocker, blocked the effects of morphine microinjected into NRM but not effects of morphine injected into NRPG. Phenoxybenzamine partially blocked the effects of morphine injected into NRPG but not the effects of morphine injected into NRM. These results show that both nuclei are sensitive to morphine, exert their effects by different synaptic mechanisms and that NRPG does not make an appreciable contribution to the analgesia produced by systemically administered morphine.