Details are given for a method of preparation from 3:5-diiodo-L-thyronine of triiodothyronine labelled in the 3' position with "1"I. Radioactive triiodothyronine appears to be much more stable in solution at pH 8-9 than labelled thyroxine.The labelled compound was injected into rats, with or without simultaneous administration of butyl 4-hydroxy-3:5-diiodobenzoate (BHDB), and the radioactivity of the exereta was measured.Administration of BHDB caused a marked reduction in the output of urinary iodide in intact rats after a single injection of labelled triiodothyronine at a number of different dose levels. This is interpreted as being due to interference with the deiodination of this compound.