Retardation of testicular growth, which occurs in hypophysecto-mized rats following implantation of hypophyses previously treated with estrogens, can be prevented by simultaneous admn. of chorionic gonadotrophic hormone. In the present expt., hypophysectomized female rats were used as recipients. About half of the donors, male and female rats, were treated daily with 0.2 to 0.5 mg. of estradiol monobenzoate for 12-16 days, while the others remained untreated. The recipient female rats were hypophysectomized and unilaterally ovariectomized. On the same day, one hypophysis was implanted intraperit. and the animals injd. with 2.5 I.U. of chorionic gonadotrophin (pregnyl, Organon) twice daily for 4 days. The recipients which received one normal hypophysis and pregnyl had an avg. ovarian wt. of 45 mg. while those receiving one hypophysis, previously treated with estrogen, and pregnyl had an avg. ovarian wt. of 28 mg.