Whole body scanning and radioautography of plaice injected with 125I failed to show extra-pharyngeal sites of thyroid activity. The constituent iodoamino acids of thyroid tissue and of blood plasma have been identified by thin-layer chromatography of the parent compounds and of their derivatives, the preparation of which is described. Thyroid tissue has been found to contain 3,5,3\m='\-tri-iodothyronine(T3), thyroxine (T4), 3-iodotyrosine (MIT) and 3,5-di-iodotyrosine (DIT). T3 and T4 were the only iodoamino acids detected in plasma.