On the basis of our previous investigations concerning the influence of some central nervous system structures on the immune reaction, we have shown the existence of a brain regulatory system for immune response (BRSIR). We have investigated this BRSIR using small electrolytic lesions placed in different brain areas of male rats, followed by two methods to evaluate the effect of the lesions on the immune response, the method of delayed skin hypersensitivity and the method of utilization of 3H-thymidine for synthesis of DNA after injection of muramyl dipeptide, a very potent immunostimulating compound. The experiments were performed on rats of the Wistar strain. Lesions were placed from the spinal cord through the brain stem to the cerebral cortex. The results suggest that the following structures constitute parts of the BRSIR: medial frontal cortex (areas Cg 1–3), subnucleus basomedialis and centralis of the amygdala, subnucleus medialis and dorsolateral of the nucleus parabra-chialis, lateral reticular formation (nucleus parvocellularis - mainly areas corresponding with aminergic groups A1–7), part of the raphe reticular formation (nucleus raphealis dorsalis and nucleus linearis – mainly areas corresponding with serotoninergic groups B6–8), and the spinal cord.