This study was directed at determining whether morphine's immunomodulatory effects are mediated through the periaqueductal gray (PAG). The initial study showed that microinjection of morphine (0.0, 0.4, 4.0, or 40.0 micrograms/rat) into the lateral ventricle induces pronounced dose-dependent reductions in lymphocyte proliferation to T- and B-cell mitogens, natural killer cell cytotoxicity, and the production of interleukin-2 and interferon-gamma. In contrast, microinjection of morphine (0.0, 0.004, 0.04, 0.4, or 4.0 micrograms/rat) into the caudal aspect of the PAG induced dose-dependent alterations in natural killer cell cytotoxicity, but had no effect on lymphocyte proliferation or cytokine production. These results indicate that opioid receptors in the PAG are involved in the regulation of natural killer cell activity, but are not associated with morphine's effects on proliferation or cytokine production. A subsequent study showed that the effect of morphine in the PAG is restricted to the more caudal aspects of the PAG because microinjections of morphine into the rostral aspects do not result in any alteration of immune status. To determine that the activation of opioid receptors in the PAG is not only sufficient, but is required for morphine's effects on natural killer cell activity, N-methylnaltrexone was administered into the PAG (0, 0.0001, 0.001, or 0.01 micrograms/rat) before the systemic administration of morphine (15 mg/kg), a dose that induces pronounced alterations of natural killer cell activity. The results showed that the administration of N-methylnaltrexone directly into the PAG antagonized morphine's effects on natural killer cell activity, which indicate that activation of opioid receptors within the PAG are required for morphine to alter natural killer cell activity. Collectively, this study showed that activation of opioid receptors within the more caudal aspects of the PAG are required for morphine to induce alterations in splenic natural killer cell activity. The results also suggest that other brain regions are responsible for morphine's effect on lymphocyte proliferation and cytokine production.