The animals used in this study were male Wistar rats, 1.5 month old. All the animals were randomly divided into 12 groups, with 50 rats per group. The obser vation period after injection of a single dose of ,125,I of ,13l,I was two years blood sample were drawn from every animal heart under ether anaesthesia. The levels of T,3, T,4, and r-TSH in serum were assayed and the relative thyroid weight (i. e. ratio of thyriod weight and b. w.) was estimated. All of the parameters just mentioned above, compared between ,125,I-and ,131,I-groups.,The experimental results showed that, the relative thyroid weight in the experimental groups decreased to the level which is 80% or 60% of control. The Ratios of equivalent dose of ,125,I and ,131,I were 1.5 and 1.2 respectively. From the biological effect point concerned, comparing the serum levels of T,3, T,4, and r-TSH in ,125,I and ,131,I groups, the ratios of equivalent dose are 2.1, 19 and 4.5 respectively.,As mentioned above, the results indicated that for the ,125,I required 1,2-19-fold as many doses as ,131,I did.