The incorporation of Zn65 into various organs of mice 5 days after single X-irradiation at dose levels of 300 r, 600 r, 900 r, and 1200 r was studied. The time course of isotope metabolism in the organs of irradiated animals at a dose level of 900 r was also studied. After irradiation the total Zn65 content of the spleen of irradiated mice decreased and the isotope concentration increased as compared with the 2 control groups. This phenomenon was not fully explained by a decrease of food intake. The total Zn65 content of the testis decreased after irradiation, but the concentration of isotope did not change. A decrease in appetite after irradiation could not explain these results. The Zn65 metabolism in the kidney of irradiated mice was apparently the same as that of animals receiving limited food. It is suggested that the zinc metabolism of the kidney of mice is resistant to whole-body X-irradiation at the doses given. The Zn concentration of the liver of mice increased on whole-body X-irradiation, and the content became slightly more than that of normal animals. It is suggested that the zinc metabolism of the liver of mice is sensitive to whole-body X-irradiation. The Zn65 concentration of the pancreas increased significantly as compared with that in the normal and diet control groups 5 days after irradiation.