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Neuroimmunological research has indicated that hypothalamic structures play an important role in the modulation of immunoresponsivity. We report that electrical stimulation of the anterior hypothalamus decreases the concentration of Immunoglobulin G (lgG) in rat serum. Male Sprague-Dawley rats were randomly assigned to either an anterior or dummy electrode group. Following the determination of baseline IgG concentrations animals were placed in an operant chamber where electrical stimulation was delivered to animals in the anterior group. Animals in the dummy electrode group were placed in an operant chamber but received no brain stimulation through the attached cable. IgG concentrations were determined for each animal 3, 6, 12, and 24 hours following each session in the operant chamber. Poststimulation measurement periods were counterbalanced across animals to control for possible order effects. A significant decrease in IgG concentrations was found between animals in the anterior group and animals in the dummy electrode group. An analysis was also performed to assess the effects of repeated measurements on IgG concentrations. As compared to the dummy electrode group, the anterior group displayed significantly higher IgG concentrations as a function of the number of measurements. The results of the present study indicate that anterior hypothalamic activity can change IgG concentrations without antigenic challenge and the relevance of this finding as applied to current research in Health Psychology is discussed.