In the rat a reciprocal relationship between the secretion of luteotropin and that of gonadotropins has been reported by a number of workers. In persistentdiestrous rats secured by 30 daily injections of estradiol from the day of birth, the ovaries remained infantile and the vaginal smear was characterized by a state of constant diestrus. Corpora lutea induced in such animals by injections of gonadotropins did not secrete progesterone spontaneously, at least in an amount sufficient to counteract the cornifying effect of excessive doses of estradiol. However, if the adenohypophysis of these rats was autografted under the kidney capsule, corpora lutea secreted enough progesterone to induce strong mucification in the vaginal epithelium in the presence of excess estradiol. These findings indicate that in persistent-diestrous rats, although the production of LH-releasing factor by the hypothalamus and consequently that of LH by the adenohypophysis are impaired (Arai, 1963), the secretion of luteotropin is inhibited by the hypothalamus.