Rats were submitted to an iodine deficient diet during 6 months. At the end of this treatment, normal iodine diet was given to the animals during 16 days. For the first two days of iodine refeeding, thyroidal iodide was strongly stored, which is explained by a very stimulated iodide pump and by an iodination blockade. After this latency, the thyroidal iodide promptly decreased whereas iodide pump was always activated, and iodination still inhibited. A retroinhibition by thyroidal iodide on its active transport is postulated.