The changes in the general condtions and blood have been observed in the rats receiving oxygen iehalation for 12 hours a day. In the first stage when the animals are laced in an oxygen chamber, they become less active and sooner or later they fall into a sleeping state with a reduced respiation frequency without taking food. After about 20 days' training even in the oxygen atmosphere they show a little movement with some activity and a little increase in appetite. After about 30 days in the oxygen chamber they begin to act almost as if they were outside the chamber with a slight loss in body weight. Hemoglobin content of blood decreases day by day during the first 20 days, reaching a new equilibrium, 20-30 per cent of the original value, which is kept up for the following 40 days without any change. Red cell number continues to fall reaching the minimum value after about 20 to 30 days, 90 per cent of the original number, and increase gradually reaching the original level about 2 months after the initiation. The reticulocyte number increases day by day reaching the maximum of 52.3 per cent after 10 days and keeps this level till the end of the experiment, i. e for two months. Leukocyte number is kept in a normal range throughout the experimental days. From the above experimental data the author concludes that the decrease in hemoglobin content seems to be due to the toxic action of oxygen itself but far from the adaptation syndrome, as only 2 per cent or less decrease in hemoglobin can be expected from the theoretical calculation in the oxygenation rat of hemoglobin.