A study has been made of pituitary grafts placed into hypophysectomized adult rats under the median eminence of the tuber cinereum, under the temporal lobe of the brain and into the hypophysial capsule. Good union was obtained in all three sites, the grafts becoming richly vascularized and remaining viable for long periods. Grafts placed under the median eminence acquired vascular connexions with the primary plexus of the hypophysial portal vessels, whereas those under the temporal lobe of the brain and in the hypophysial capsule were not supplied by the portal system. Grafts placed under the temporal lobe of the brain and in the hypophysial capsule had greatly diminished, if any, function, but many of those placed under the median eminence of female rats had normal function as judged by ( a ) oestrous cycles, pregnancy, milk secretion (milk withdrawal from the nipple by the young was impossible, owing to neural lobe deficiency as shown by replacement therapy), ( b ) ovarian and adrenal weight and histology, ( c ) thyroid histology. Anterior pituitary tissue obtained from immature donors grafted into adult recipients showed hastened development. Similar tissue obtained from adult male donors grafted into adult female recipients was capable of maintaining normal oestrous cycles and pregnancy. A few experiments using male recipients indicated similar results concerning structure and function of the grafts. These results offer strong support to the view that the secretion of anterior pituitary hormones is under hypothalamic control, mediated by the hypophysial portal vessels.