SUMMARY: It is shown that anterior pituitary tissue grafted into hypophysectomized rats and lacking any connexion with the median eminence does not retain any somatotrophic or gonadotrophic activities, but has some adrenocorticotrophic (ACTH) and thyrotrophic (TSH) activities. This conclusion is based on: (1) the partial maintenance of thyroid and adrenal weight; (2) the results of studies on 131I uptake and on TSH concentration in the grafted pituitaries; (3) the persistence of significant adrenocorticotrophic responses to Pitressin, lysine-vasopressin and Guillemin's fraction D; (4) the presence of histological and histochemical signs of activity in the grafted gland. The subnormal levels of secretion observed in the pituitary grafted into sites far removed from the sella turcica, seem to depend on the lack of some neurohumoral stimulus which normally activates the intact pituitary via the hypophysio-portal vessels. Evidence is given that vasopressin may be the hypothalamic neurohumor involved in ACTH secretion, and that the ACTH-releasing activity of Guillemin's fraction D may be accounted for by its vasopressin content.