Hypophysectomy markedly depresses thyroid function as evidenced by the depression of thy-roidal content of 127I, 131I uptake, of plasma PB131 (protein-bound iodine], and of modified ratios of radioiodo amino aicds in thyroidal io-doprotein (depression of the thyroxine level, correlated to an increase of MIT [monoiodotyrosine]). Thyroidal horomonogenesis appears, therefore, to be maintained but slowed down. In drakes with autografted pi-tuitaries and with transected portal veins, there were less marked, though evident, alterations of thyroidal I metabolism: thyroidal 127I content is increased, whereas the 131I uptake is paranormal; PB131I is depressed 24 hr. after 131I injection. Alterations of the ratios of labeled iodoamino acids of iodoproteins analogous to those found after hypophysectomy, are apparent 24 hr. after radioiodine injection though of less magnitude. In birds in the basal state, functional autonomy of the thyroid with respect to the hypophysis is low but evident, whereas that of the thyrotrophic activity with regard to the hypothalamic control is much more developed.