After administering Zn(65)O and Zn(65)Cl(2) to guinea pigs the author investigated the difference in the distribution of these isotopes and also the distribution of Zn(65)Cl(2) in serum fractions and obtained the following results. 1. (a) The in vivo distribution of Zn(65)O was high in the lung, liver, heart, kidney, pancrease and intestine 15 minutes after its administration, in the lung, liver, spleen, pancreas, kidney and int stine two hours later, and two weeks later in the lung, large intestine, heart, liver, spleen, intestine and pancreas, the activity being in the order of the organs mentioned. In addition, the radio-activity detected in the urine is minute as compared with the quantity of Zn(65)O injected. (b) The in vivo distribution of Zn(65)Cl(2) after five hours its administration was found to be in the order of the liver, intestine, pancreas, large intestine, kidney and lung, showing a distinct difference from the the case of Zn(65)O administration. 2. The radio-activity of zinc ions combined with serum protein fractions after the Zn(65)Cl(2) administration was found in β-globulin, γ-globulin, albumin and α-globulin, the activity being descending order mentioned, and the relative specific activity (activity/area under optical density curve) was in the order of γ-globulin (12.5), β-globulin (6.9), albumin (2.3), and α-globulin (1.1).