Unilateral cryptorchidism was produced in 100 g male rats. Some of the - animals were hypophysectomized 2–3 days before the testicular operation. Half the intact and half the hypophysectomized rats were injected with 100 IU HCG daily from the time of operation. Three weeks later the ability of the cryptorchid testes to convert progesterone to 17α-hydroxyprogesterone, 4- androstenedione and testosterone was compared with that of the scrotal testes of the same animals. In all cases the activities of the scrotal testes were significantly greater. Although hypophysectomy caused a marked reduction in the enzymic activity of the scrotal testes, that of the abdominal testes was reduced even more. The scrotal testes responded to HCG with marked increases in activity in both the normal and hypophysectomized groups. The cryptorchid testes of the hypophysectomized rats showed an increase from their very low levels, but those in the intact rats did not respond. Apparently there is a limit to the response of the cryptorchid testis which is much below that of the eutopic organ.