The whole-body irradiation of adult Wistar rats with a single dose of 800 r X-rays intensifies the lymph node changes and the decrease of the colloidopexic capacity of hepatosplenic macrophages established by the "reverse" administration of sexual hormones. The modifications observed after estradiol administration to the male rat are more marked after a 6 months' interval than those caused in the same interval by testosterone in the female rat lymph nodes.