Further studies of the metabolism of thyroxine and 3,5,3'-triiodothyronine in the guinea-pig.

1. Thyroxine labelled with (125)I and triiodothyronine labelled with (131)I have been administered simultaneously to guinea-pigs and their metabolism studied by whole body counting and measurement of radioactivity in urine, faeces, and blood.2. The half-life of triiodothyronine in the body is about 19 hr, significantly less than that of thyroxine (41 hr).3. After triiodothyronine administration, an unidentified iodinated compound appears in the blood which complicates the estimation of the half-life of this hormone by measurement of the radioactivity in peripheral blood.4. A previous report, based on measurements of blood radioactivity, that the half-lives of the two hormones are similar in the guinea-pig was not confirmed.