Observing the histological changes of the internal organs of the mice, having daily oral administrations of arsenious acid and simultaneous oral infection of Ectromelia virus and of the mice infected Ectromelia virus 4 days after the beginning of the arsenious acid administration, in compared with those that had daily arsenious acid without the infection, and the following results were obtained. 1. In the cases had the continuous administration of arsenious acid administration, in compared with those that had daily arsenious acid without the infection, and the following results were obtained. In the liver of the same cases, degeneration and necrosis of the liver cells and nuclei were marked, on the other hand, regenerations of the liver cells in association with round cell infiltrations were noted. There were extensive parenchimal impairments in the kideny and heart as well. 2. In the cases with Ectromelia virus infection on the 4 th day after the beginning of the arsenious acid administration, much severe vascular damages, and degenerations and necrosis of the liver parenchyma especially marked in the peripheral areas of the lobules were observed There were more extensive parenchymal imparments in the kidney and heart. 3. The cases with both Ectromelia virus infection and the administration of the arsenic acid presented the pathological findings summed up both of damages but especially marked in the vascular regions. In the cases with viral infection following the arsenious acid intoxication, the vascular damages due to the intoxication gave some influences upon the further pathological changes. 4. The vascular damages caused by the arsenious acid intoxication appeared to be essentially similar to those of Ectromelia virus infections.