Although research has linked central nervous system activity with changes in immunoresponsivity, research on the possible role of the central nervous system in altering a specific class of antibody is lacking. This study was an investigation of the possible relationship between anterior or medial hypothalamic functions on Immunoglobulin G. concentrations in rat serum. Thirty-six male albino rats were randomly assigned to three groups of equal size. Animals within the anterior hypothalamic group received bilateral electrode implants in the anterior hypothalamus while animals in the medial hypothalamic group received electrode implants within the ventromedial area of the hypothalamus. A control group received bilateral electrode implants within the lateral hypothalamus. Electrical brain stimulation was administered to animals in both experimental groups. Control animals spent a comparable time in an operant chamber but did not receive electrical brain stimulation. Following brain stimulation of animals within the experimental groups, Immunoglobulin G. concentrations were determined for all groups 3, 6, 12, and 24 hours post-stimulation sessions.