Female rats of the Fischer 344 strain at ages of 1, 4 and 9 weeks were exposed to 131I intraperitoneally with activities of 0.38, 1.03 and 3.42 kBq per gram of body weight under the condition of iodine deficiency. The absorbed doses in the thyroid increased linearly depending on the injected activities. Irradiation at 1 week old caused heavier exposure than those at 4 and 9 weeks old by 7.5 and 7.7 times, respectively; however, damage of the thyroid tissue was more obvious in the 4-week-old groups than in the 1-week-old groups. The absorbed doses in the total body were proportional to the square root of the injected activities. The one-week-old groups were exposed more heavily than the 4- and 9-week-old groups by 3.6 and 4.7 times, respectively, shown by the slow excretion of 131I with the values of effective half-life of 131I activity (Teff). An IDD-treatment was not so effective to enhance the 131I absorption in the total body, as in the thyroid. No matter how the iodine concentration in the blood changed, the 1-week-old groups could not react to normalize the level. We drew standard curves, which enabled us to estimate the absorbed doses in the thyroid and the total body in the case of the injected activities of 131I for the newborn, pubertal and adult rats.