The absorption and enterohepatic circulation of thyroxine and its excretion product, Compound U, were studied in the rat. Bile obtained from rats injected with 100 to 125 [mu]g. of I131 labeled L-thyroxine was infused slowly, via the bile duct, into the duodenums of recipient rats. The I131 in the infused bile, which was largely in the form of Compound U, was absorbed to the extent of 25 to 35% within 65 hrs. 5.5 to 14.7% of the administered I131 appeared in the bile secreted by the recipients. I131-labeled L-thyroxine was added to nonradioactive bile and infused slowly, via the bile duct, into the duodenums of recipient rats. The absorption of the thyroxine iodine varied from 41 to 68%. The I131 which appeared in the bile secreted by the recipient rats varied from 17 to 31% of the administered I131. I131-labeled Compound U, isolated by elution from filter paper chromatograms, was added to nonradioactive bile and infused slowly into the duodenums of recipient rats. The absorption of the iodine varied from 24 to 40%. The I131 which appeared in the bile of the recipient rats varied from 9.6 to 20.3%. Compound U is less readily absorbed from the gastrointestinal tract than is thyroxine. The conversion of thyroxine to Compound U in the rat therefore permits the animal to eliminate excess thyroxine somewhat more efficiently.