The changes of various organ tissues were hirtologically observed per day on the intraperitoneal inoculation of ectromelia virus to mouse. And the results were as follows. 1. The changes of various organ tissures found at first was the findings accompanied by the functional and organic changes of blood vessel, the changes of lymph vessel system, reticuloendothelial cell system and connective tissue system, and was the remarkable appearance of perilobular tissue reaction including the wandering cells, and then the change extended to the parenchymal tissue and the various organ tissues became showing the special pathologic picture respectively. 2. The living period of animal was contracted with the increase of inoculated virus dosis. And there was quantitative difference, but no qualitative difference on the histological change at that time, and the convalescent process was not found at all and the infiltration of round cell was also very few in the cases with the exceedingly short living period. 3. The histological change was especially remarkable in the liver, spleen and bone marrow, and it was hemorrhagic alveolitis in the lung and the pathologic picture of nephronephritis in the kiduey. 4. The changes at the maximum stadium of the cases inoculatied the extremely high dosis of ectromelia virus were similar to those of the histological pictures of various organs in fulminant hepatitis of human and the changes at the maximum stadium of the cases inoculated the relatively high dosis of ectromelia virus were similar to those of the histological pictures of various organs at the maximum stadium of acute hepatitis in human. And the histological pictures of liver at the period from the inoculation to the maximum stadium and those of the animals being relieved of death were also similar to those, examined by the liver biopsy, at the period from the beginning to the maximum stadium and at the convalescent stndium in human hepatitis. 5. The pathologic pictures of mice with the inoculation of ectromelia virus were very similar to those of epidemic hepatitis in human and it was thought that both of them were based on the interlobular reaction.