This study describes the effects of electrolytic lesions of different brain structures on the lymphatic tissue of the rat. Significant involution of the thymus occurred in all rats 32 days after electrolytic damage of the hypothalamus, reticular formation, thalamus, superior colliculus, caudate nucleus and amygdaloid complex. However, only the thymus of hypothalamus-lesioned, reticular formation-lesioned and superior colliculus-lesioned animals exhibited drastic changes in terms of a depletion of lymphocytes, reduction of the cortex and disappearance of the cortical-medullar junction. Plasmacytic reaction was regularly observed in thymuses of all immunized and nonimmunized brain-lesioned rats. The cellular architecture of the spleen and lymph nodes was affected only in hypothalamus-lesioned animals. The principal findings were a decrease in the number of lymphocytes and plasma cells, and absence of germinal centers. The results are discussed in the light of the interconnections between the nervous, endocrine and lymphatic system.