The corticosterone content of the adrenal glands was determined in 21-day-old rat fetuses: before and after encephalectomy or decapitation (hypophysectomy); after ACTH treatment of the encephalectomized or decapitated (hypophysectomized) fetuses; and after administration of crude extracts (0.1 NHC1) of hypothalamic or cortical tissue from 20-day-old rat fetuses. A peak in the corticosterone content of the adrenals was observed 10 min after ACTH injection to encephalectomized or decapitated fetuses. A rise in corticosterone concentration was noted 5 and 10 min after the encephalectomized fetuses were given an injection of hypothalamic extract. This extract was devoid of appreciable ACTH activity when tested in decapitated fetuses. Cortical extract was inactive in encephalectomized or decapitated fetuses. These data suggest that fetal hypothalamic extract contains some CRF activity and that the fetal pituitary gland is responsive to the CRF.