The effects of neonatally administered sex hormones on the growth and hormone responsiveness of the sex accessory tissues of the adult male rat were investigated. If intact neonatal male rats are treated with estradiol diproprionate, the size of the ventral prostate in the adult animal at day 70 of age is less than 10 per cent of the untreated control value. If androgens are administered to intact neonates, the size of the ventral prostate at day 70 is one-third to one-half of the untreated control values. The androgenic response of these abnormally small prostates was tested by treating these adult animals with testosterone propionate daily from day 70 to day 83. Toward ventral prostates of the animals treated with estrogens in the neonatal period show a very limited response to exogenous androgens at adulthood. In contrast, the ventral prostates of the animals receiving testosterone propionate or dihydrotestosterone propionate in the neonatal period were fully responsive to exogenous androgens. In addition, the ability of the adult prostate to respond fully to androgens requires the presence of the testes in the neonatal period. Comparing neonatal castrates with intact neonates, it was shown that the administration of androgen in the neonatal period was only partially effective in restoring the androgenic responsiveness of the adult ventral prostate. This suggests that other testicular factors, or the presence of testosterone at other time periods before the onset of puberty, may be necessary for the normal androgenic induced response of the adult sex accessory tissues.