SUMMARY: Orchidectomy of Balb C mice aged between 4 and 6 weeks delayed the normal rate of thymic involution. Testosterone replacement after orchidectomy prevented this relative thymic enlargement and testosterone supplements given to intact mice accelerated the normal rate of thymic involution. When hypopituitary, dwarf mice were given somatotrophin and thyroxine alone there was no significant difference in the thymic weights of control or orchidectomized mice. When replacement therapy included gonadotrophin, orchidectomy was associated with significant thymic hypertrophy. Passive transfer of serum from previously orchidectomized mice did not change the thymic weight of normal recipient mice. These experiments suggest that testosterone exerts a constant moderating effect on the thymus and the immunological effects of orchidectomy are due to the direct effects of androgen withdrawal.