Adult hypophysectomized male rats were injected daily with 2 mg pregnenolone for 30 days. In these rats the testosterone concentrations in peripheral blood (0.5 ng/ml) and testes (46 ng/g) were appreciably lower than in intact rats (4.1 ng/ml and 167 ng/g, respectively). In hypophysectomized animals treated with pregnenolone the testes and epididymides were much better maintained than the prostates and seminal vesicles. High concentrations of dihydrotestosterone were found to be present in the head (12 ng/g) and body (8 ng/g) of the epididymis of these rats. Although the number of spermatozoa in the distal part of the epididymis of pregnenolone-treated hypophysectomized rats was only 23% of the number found in intact control animals, the spermatozoa were fertile.