The pathological changes of the visceral organs i.e. the liver, spleen, lung, meso-nephros and gonad of the Rous virus infected chick embryo have been investigated. Theorgan tissues were removed at various intervals from 6 hours to 10 days after virusinoculation. They were fixed in Carnoy's solution and the sections were stained withmethyl green-pyronin. Some of them were stained with Alcian blue-hematoxylin for acidmuco-polysaccharide and toluidine blue for metachromatic reaction. Sequential histologi-cal examination on the process of pathological changes and their histochemical reactionsof the visceral organs revealed the following facts: Hemorrhagic lesions appeared in 60.3% of inoculated host livers from the 5-7 daysonward. Distinctive focal necrobiotic change of the liver cells, destruction of the endo-thelial cells, distention, hyperaemia and extravasation of the blood vessels and sinusoidswhich characterized the hemorrhagic lesions had been observed. Other pathologicalchanges such as the distortion of the hepatic cords, nuclear pyknosis and loss of stainingreaction of the liver cells in the hemorrhagic lesions were also found. Hemorrhagiclesions also appeared in the mesonephros, lung, intestine and heart of some of the em-bryos having received virus inoculation. In some of the embryos, focal sarcoma-like proliferation or tumor-like nodules werefound in the liver, lung and mesonephros after 7-9 days of virus inoculation. Alcianblue staining of the nodules or foci demonstrated the presence of acid muco-polysaccharidereaction in the ground substance, indicating an early phase of growth of Rous sarcomain the infected mesodermal tissues in these organs. It was observed that a certain number of pyroninophilic round cells occurred in allof the visceral organs of the host. They formed aggregates around the blood vessels orscattered in the mesenchymal tissues. Sometimes these cellular aggregates were quite ex-tensive, forming large patches. The incidence of these cellular aggregates or patches inthe liver, spleen, lung, meseonephros and gonad were 52.57%, 39.37%, 31%, 41%, and37.77% respectively. The cells in these patches had an Alcian-blue positive reaction ofacid muco-polysaccharide and metachromatic toluidine-blue staining. Both of the stainscould be removed by 3--5 hours treatment with testicular hyaluronidase. These cellswere identical with the cells which we had found previously in the CAM-tumor inducedby the Rous virus, and were considered also to be belonging to the basophilic myelocyteseries. Their possible role in the pathological changes in the visceral organs was dis-cussed.