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The age at which rats are most sensitive to prepubertal gonadectomy, in terms of thymic growth, was investigated. Gonadectomy enhanced thymic growth at each age; the greatest difference in thymic weight between gonadectomized and intact animals occurred in male rats gonadectomized at 5 weeks of age (64%) and in female rats gonadectomized at 4-5 weeks of age (43-46%). The effect of various synthetic sex steroids on growth and lymphocyte populations of the thymus in gonadectomized rats was examined. Diethylstilboestrol, 1 mg per animal, inhibited thymic growth by more than 35% in both males and females. Ethinyloestradiol, 40 micrograms per animal, inhibited thymic growth by 26% in males but by only 4% in females. Fluoxymesterone, 10 mg per animal, inhibited thymic growth by more than 46% in both sexes. Norgestrel, 12 micrograms per animal, had no effect on thymic growth. The synthetic steroids that significantly inhibited thymic growth decreased the intensity and altered the localization of staining for total T cells (antibody clone MRC OX 19), T helper cells and macrophages (W 3/5), T cytotoxic/suppressor cells (MRC OX 8) and B cells (MRC OX 12).