The involvement of the nucleus accumbens in neuroimmunostimulation was demonstrated during activation of opioid mu receptors with the selective agonist DAGO (100 microg/kg); single doses of this agent to sham-operated (control) Wistar rats induced significant increases in the numbers of direct IgM antibody-forming and total rosette-forming cells after immunization with sheep erythrocytes. Bilateral electrolytic lesioning of the nucleus accumbens in rats led to sharp decreases in the intensity of immune responses; there was no immunostimulation after administration of DAGO to these animals. These data provide evidence for the involvement of the nucleus accumbens in the process of immunomodulation and for the importance of opioid mu receptors in the nucleus accumbens in the stimulation of immunogenesis.