The present study assessed the involvement of opioid receptors both in the central nervous system (CNS) and in the periphery (i.e., on immunocytes) in the immune alterations produced by acute morphine treatment in rats. The first experiment showed that the in vitro suppressive effects of morphine on the mitogen-stimulated proliferation of splenic and blood lymphocytes are produced only by a very high concentration of morphine and are not naltrexone-reversible. These results suggest that the in vitro immunomodulatory effects of morphine are not mediated by classical opioid receptors on immunocytes. A second experiment showed that s.c. doses of N-methylnaltrexone that do not gain access to the CNS, as determined by the tail-withdrawal assay, do not antagonize the suppressive effects of a single, s.c. injection of morphine on the mitogen-stimulated proliferation of splenic and blood lymphocytes, splenic natural killer cell activity and the production of interferon-gamma by stimulated splenocytes. Only a high s.c. dose of N-methylnaltrexone that does gain access to the CNS, as determined by the tail-withdrawal assay, blocks morphine's immunomodulatory effects. A third experiment demonstrated that N-methylnaltrexone is 4 to 5 log units more potent in antagonizing most of the immune alterations produced by a single, s.c. injection of morphine when administered i.c.v. than s.c. Taken together, the results of the present study strongly suggest that CNS opioid receptors play an important role in the immune alterations produced by acute morphine treatment in rats.