Weanling and adult white Smith rats were exposed to 500 r whole body roentgen irradiation and the blood constituents were analysed for a period of 70 days. The reaction time and recovery rates of the various blood constituents were compared for the weanling and adult rats. In addition to this examination, I attempted to observe whether the pure Ginseng powder possess the effect of protective action to x-ray irradiation. 1. Weanling rats responded more quickly and more drastically to whole body roentgen irradiaton by leukopen ia than did the adults, but they also recovered better after the irradiation insult. 2. Lowest Iymphopenia occurred by the first day in the weanlings and by the third day in the adults after irradiation. 3. Both in adult and weanling rats lost a high percentage of their neutrophiles by third days similary after irradiation, but weanlings recovered better than did adult at 49 dats examination. This indicates more destructive changes of neutrophiles of adult rats. 4. Erythrocytes were originally higher in adults than weanling rats. 5. Hemoglobin was originally higher in adults than weanling rats and both responded to whole body roentgen rradiation with a maximum drop at 14 days. 6. Platelets were originally higher in wealing rats than adult rats and responded to whole body roentgen irradiation with a maximum drop at 7 days and platelets in both adults adn weanlings showed over compensation at 49 days examination after irradiation. 7. Severe leukopenia was noted before death of the irradiated rats. 8. In the ginseng administerd rats, there was no evidence of prolonged life span and no evidence of better recovery was found.