After injn. of I131 -labelled thyroxine (30 mc/12 mg.) into rabbits, concn. of I131 occurred in the pituitary. No such concn. occurred after KI131, whose distr. was like that of NaCl. Thyroxine was isolated from the hypophysis by isotope-dilution technic. Concn. 2 hrs. after injn. was 2 times plasma concn. When 250 or more [mu]g. of thyroxin were injd., concentrating power dropped and a maximum of 10-20 x 10-3 [mu]g. of thyroxine were found in the pituitary. The thyroxine concentrating power of the hypophysis is exerted altogether by the posterior lobe. The cat, guinea pig, rat, and cock all failed to concentrate thyroxine in their pituitaries; the monkey (Cercopithecus) showed slight activity (posterior lobe/blood = 1.67) and the rabbit was most active (posterior lobe/blood = 4.38). Testicle, salivary glands, myocardium, adrenal and lymph nodes fail to concentrate labelled thyroxine, but radioactivity was found in bile and urine.