Histopathological changes of various organs of the mice after intra-peritoneal injections of radioactive iodine (131I) were experimentally observed. Sixity healthy female mice, weighing average 25gm, devided into 6 groups, were used. The various doses of 131I were injected intraperitoneally at different intervals. The histopathological changes after these treatments were observed in organs such as thyroids, parathyroids, livers, kidneys and gonads. Following were the results;1) Thyroid:In the group A given 131I with a single dose of 10μC per gm body weight, it was observed that the protoplasms of follicular epithelial cells were destroyed, the nuclei were expanded or dissoluted, showing pyknotic changes of nuclei and vacuolizations of protoplasms. In the group B given 131I with a single dose of 5μC per gm body weight, hyperemias, hemorrhages and hyaline degenerations in the whole area were observed. In the group C given 131I with 3 doses of 2.5μC per gm body weight every week, the thyroid parenchyms were destroyed and epithelial cells of varing size were observed in the filbrinous tissues. In the group D given 131I with 6 doses of 0.5μC per gm body weight every week, some destroyed follicles and new borne follicles were observed. But the histopathological changes resemble the follicles of the normal thyroid gland. In the group E and F given 131I with 8 and 10 doses of 0.2μC and 0.01μC for each group per gm body weight every two days, both pyknotic changes of nuclei and cytoplasmic vacuolizations of the follicular epithelia, hypertrophies of follicles and abnormal irregular follicular structures were observed, and in the group F, lymphocytes appeared around the thyroid glands. 2) Parathyroid: In the group A, hyperemia, proliferations of connective tissues, karyorrhexes and vacuolizations were observed. In other experimental groups, no particular pathological change was observed. 3) Liver:The degnerative changes and acute or chronic inflammatory changes were observed in proportion to the amount of 131I injected. Atrophies of the liver cells, dilatations of sinusoids, hyaline degenerations and necrotic pictures were observed. 4) Kidney:In the group A, congestions and infiltrations of mononuclear cells and granulocytes were observed around the cortical arteries, and in the group B, the degenerative changes of cortexes, and in the group C and D, hydronephrotic changes were observed respectively, and hyaline degenerations were partially observed. 5) Gonad:In the group A, the follicles were degenerated. The ova in the follicles showed irregular figures. The changes in the group B were almost the same as in the group A, but the changes were mild. In the group C, the destructions of whole ova, the hypertrophies of ovarian follicular. membrances and pyknotic changes of nuclei were observed, In the group D, the pathological changes were similar to that of group C, but mild in the grade. In the group E, almost none of ovarian follicular fluid was observed, and in the group F, the tissue pictures were almost similar to that of the normal group.