Tritiated dihydrotestosterone ([3H]DHT) binds with high affinity to a single class of limited capacity sites in cytoplasmic preparations from rat and mouse thymus glands. The affinity of these bindings sites for [3H]DHT (Kd at 4 C, 1 nM) and their specificity [methyltrienolone (R1881) greater than DHT greater than testosterone much greater than estradiol = progesterone greater than promegesterone (R5020) much greater than aldosterone = cortisol] are characteristic of androgen receptors in classical target tissues. The affinity of thymic androgen receptors for [3H]DHT did not differ with age, sex, or species; the concentration of androgen receptors was higher in males than females and higher in older compared with younger animals. Castration of male mice was followed by marked (approximately 2-fold) thymic hypertrophy; the concentration of androgen receptors fell to approximately one third the control value as a function of organ weight, but did not change significantly as a function of thymic DNA. Chronic administration of DHT (200 micrograms/day) did not alter thymic weight or androgen receptor levels in intact male mice. In castrate mice, however, the administration of 200 micrograms DHT completely abolished the thymic hypertrophy otherwise seen post castration and significantly increased the total thymic content of androgen receptor. The administration of dexamethasone (0.6-60 micrograms/day for 5 days) caused dose-related thymolysis; on a weight basis, the concentration of androgen receptor rose markedly, suggesting that androgen receptors are confined to the subpopulation of thymocytes relatively resistant to glucocorticoids.