The aim of this investigation was to determine the radioiodine dose necessary for the ablation of the thyroid gland in male Wistar rats. By a mathematical extrapolation of the kinetics of iodine-131, the radioisotope dose in the thyroid was calculated to be 467971 or 401540 rad after an intraperitoneal administration of 1 x 0.8 mCi (29.6 MBq) or 1 x 1.0 mCi (37.0 MBq) iodine-131, respectively. Because lower radioiodine activities did not result in a complete destruction of the thyroid (controlled by scintigraphic methods), it can be concluded that about 400,000 rad are necessary for the complete ablation of the thyroid gland by radioiodine.