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Rats were hypophysectomized and their own anterior pituitary glands transplanted under the kidney capsule. Effects of chronic administration of corticotrophin-releasing factor (CRF), lysin-vasopressin and α-melanocyte stimulating hormone (α-MSH) on the adrenocorticotrophic activity of the pituitary grafts were studied using as criteria the concentration of ACTH in the grafts, resting levels of circulating plasma corticosterone, adrenal weight and also response of the graft to a challenge dose of CRF measuring plasma and adrenal corticosterone levels 12 min. later. The results show that prolonged administration of CRF has maintained synthesis of ACTH by the transplanted pituitary as well as its ability to release ACTH when acutely stimulated by CRF. These results were not duplicated by the administration of lysin-vasopressin or α-MSH in conditions identical to the ones used for studying CRF, and using doses equivalent to the vasopressor and melanophoretic activities of this particular preparation of CRF.