Radioiodide (¹²⁵I, ¹³¹I) metabolism was studied at different times in postmetamorphic, subadult and adult frogs of Xenopus laevis laevis (Daud.). The results are similar for the three animal groups, but each group shows a great interindividual variability. Thyroid uptake is slow and reaches a maximum of 6% after 96 hours. This maximum is stable over a week. 90% of serum radioactivity is formed by a free iodide fraction which is easily excreted. After 96 hours more than 50% of injected radioactivity is detected in water. In thyroid hydrolysats (pronase), after 24 hours, free iodide represents less than 5% of total radioactivity and MIT/DIT ratio is lower than one. In very short incorporation time, free iodide and MIT/DIT ratio rises. In adult frogs, thyroids does not metabolise iodide actively, it does not seem to play an important physiological function.