The beginning appearance of FSH in the pituitary glands of infant female rats was (observed by means of the fluorescent antibody technique. The presence of endogenous FSH was detected by analysis of the ovarian development Which followed injections of HCG. Rats younger than 18 days old did not show marked development of ovarian follicles although the ovarian Weights were increased; this depended on the deyelopment of the ovarian interstitial tissue by Administration of HCG. However, not only the interstitial tissue but the ovarian follicles were remarkably stimulated by injection of HCG After the rats were 19 days old, and especially at 21 days of age. Antiserum to FSH inhibited the ovarian follicle development but not the development of the interstitial tissue in response to the administration of HCG. Consequently, the authors assumed that an alteration of ovarian behavior in response to HCG would be due to the augmentation of endogenous FSH with HCG, and an active secretion of pituitary FSH starting at ...