Observing the histological changes of the internal organs of mouse following the oral administrations of 0.5 cc of arsenious acid in various concentrations, and the following results were obtained. 1. In the cases given 0.5 and 1.0% of arsenious acid orally, marked degenerations of the parenchymal cells ware observed mainly in the kidney and heart. But, vascular damages were present in every organ, especially evident in the liver and spleen; hypertrophy of stelliform cells, destruction, dilatation, hyperemia, swelling and roughness of the vascular walls. 2. In the cases given daily 0.05% arsenious acid orally, increased degenerations of the parenchymal cells in the kidney and heart, comparing with initial stadium, was noted. But, the similar vascular damages were observed in the organs especially in the liver and spleen. Moreover, the lymphatic spaces have been dilated progressively, and some serous exudates was seen in the lymphatic spaces of the liver on the 6 th day. 3. In the cases given daily 0.05% arsenious acid orally. nodules accompanied with diffuse infiltration of round cells were observed on the 6 th day. On the 7 th to 8 th day, there appeared coagulation necrosis in which central areas were almost full of polymorphnucleous leucocytes, stelliform cells and degenerative products from the liver cells. Small necrosis composed of lymphocytes and stelliform cells was seen on the 9 th to 10 th days. 4. As the histological changes of the mouse organs described above, were quite similar to those of the infectious hepatitis cases, this study would offer some suggestions to clarify the contributing factors in the pathology of infectious hepatitis.