The relation of thyroid to Br is investigated. The level of blood Br is dependent on the Cl intake. Contrary to previous reports, thyroid feeding or excessive thyroid secretion brings about a lowering of blood Br. Br conc. of normal thyroid tissue approaches that of blood while hyperplastic thyroid contains more Br than blood. If rabbits with hyperplastic thyroids are given I to involute the gland, thyroid Br returns to about the level of blood Br. The lowering of blood Br is apparently due to the diuretic and stimulating action of thyroid on the kidneys while the high Br of hyperplastic thyroid is ascribed to an inability of thyroid tissue to distinguish perfectly between I and Br when there is an I deficiency.