SummaryFollowing injection of I131 labeled L-thyroxine into intact and thyroid-ectomized mice and intact rats, tissues and excreta were investigated by radioautography of paper chromatograms of their butanol extracts. Four substances, namely thyroxine, iodide and two unidentified iodine containing substances referred to as compounds Nos. 1 and 4 were found. The same four substances were likewise found in organs and excreta taken from intact mice and rats injected 48 hours previously with radio-iodide, or in other words, after a time interval sufficient for endogenous production of labeled thyroxine in the thyroid and its subsequent distribution in the body. These findings suggest that thyroxine is metabolized to yield at least the two compounds Nos. 1 and 4 as well as iodide.