The purpose of this investigation was to study the changes in the epiphyseal plate of growing mice by histological, histochemical and electron microscopic observation through the administration of radioactive isotope I which induced hypothyroidism. To confirm hypothyroidism induced, radioimmunoassay for triiodothyronine was carried out and the morphologic changes of thyroid gland were observed by a light microscope. Of the total of eighty ICR strain of mice, fifty received single intraperitoneal injection of 20ue/gm of radioactive isotope I which were dissolved in 0.2ml of saline. Thirty received single intraperitoneal injection of only 0.2ml of saline and these mice were used as control groups. At the end of first, second, third, fourth, sixth and eighth week after injection, both saline injected and I treated mice were sacrifised. The thyroid gland and epiphyseal plate being removed were stained with hematoxylin and eosin, alcian blue and with uranyl acetate lead citrate in each for histological, histochemical and electron microscopic studies, The results obtained are as follows: 1. The thickness of epiphyseal plate was narrowed in the I treated animals. In each zone of epiphyseal plate the cell size was reduced although the cell numbers of each cell column were not changed. This means that the reduction of thickness in the epiphyseal plate was due mainly to decreased cell size and scanty territorial matrix not to decreased cell number. 2. Alcianophility was markedly decreased ln proliferative zone, indicating that acid mucopolysaccharide diminished markedly or was absent in the proliferative zone. 3. In electron microscopic examination, the number of rough surfaced endoplasmic reticulum was decreased and their cisternae were narrowed. Decreasing tendency of dilatation and sacculation of cisternae, .reduced number of attached ribosomes on the endoplasmic reticulum, and atrophy of the Golgi apparatus with diminished number of its vesicles were noted, However glycogen particlea were accumulated. 4. The number and length of each trabeculae in the metaphysis just adjacent to the epiphyseal cartilage plate were reduced markedly, and occasionally chondrocytes which were not consumed by endochondral ossification were observed in the cartilage lattice.