By observing the organs from the animals killed after the various durations of oxygen inhalation, the author reveals the following histologic changes: The inhalation of O(2), 12 houts daily, results in the heavy depostion of hemosiderin in the liver and spleen with the capillary stagnation. The severe damages in the wall of artery and arterioles are the general changes. These damages found in the vascular system will be responsible for the other histologic changes, perivascular edema and cell infiltration, thickening of the alveolar wall with the cell proliferation, localized degenerative changes of heart muscle, alternative degeneration and regeneration in the liver parenchym and the slight atrophy of glomerulus in the kidney. Active erythropoietic picture can be seen in bone marrow but decreaded number in normoblasts. The view of the author is that these changes will be the toxic action of O(2) desolved in blood plasm, which may have an active oxidizing effect. The reduction oferythrocyte number seems to be caused by the inhibition of maturation oferythroblasts, and the increased demolition of red cells in the circulating blood.