The relative affinities of the thyroid gland for 131I and 99mTcO4 and the further metabolism of these isotopes were studied in rats. 99mTcO4 is trapped to an extent almost identical to 131I in the thyroid. This process is inhibited by KClO4 and stimulated by thyroid-stimulating hormone. 99mTcO4 having been trapped is further metabolized in the thyroid gland.