An accurate method of thyroid 131I dosimetry was developed by employing the dose formulation, .**GRAPHIC**. recommended by the Medical Internal Radiation Dose (MIRD) Committee. Six-wk-old female Long-Evans rats (140 g) were injected i.p. with 0.5, 1.9, and 5.4 .mu.Ci of Na131I. The accumulated 131I activities in the thyroid were precisely determined by integrating the 131I activities/g of the thyroid as functions of postinjection time. When the mean thyroid doses derived from this method are compared to those derived from the conventional method using the well-known dose formulas, D = 73.8.hivin.E.beta.C0Teff, the conventional method overestimated the doses by 60-70%. Similarly, the conventional method yielded effective half-lives of 2.5-2.8 days (in agreement with those reported previously); these estimates were high by factors of 1.4-2.0. Apparently the biological elimination of iodide from the rat thyroid is much more rapid (up to 2.5 times) than once believed. The basic assumption in the conventional method of thyroid 131I dosimetry in the rat, i.e., that the thyroid iodide retention function is a single exponential, is invalid. Variations in animal body weight (110-190 g) of 6- to 7-wk-old animals and diurnal variation (animals injected between 0900 h and 1600 h) have no significant influence on the mean thyroid doses for a given injected activity of 131I. As expected, variation in iodide content of the animal diets (0.05-0.1 .mu.g and 1.7 .mu.g of feed) significantly altered the thyroid doses for a given 131I injected activity.