Generally it is accepted that X-ray irratiation in vivo, attacking H(2)O molecules, produces OH, O(2)H, H(2)O(2) etc., and the oxydation invoked by these by-products inhibits the enzymatic activity of the SH group and others. Bearing this point in mind, changes of ATP-ase activities in the liver, spleen, heart, kidney, and peripheral blood as well as bone marrow have been examined along with the lapse of time in the rabbits irradiated with a quite substantial amount of X-ray irradiation, namely, 2,000γ each. Chemical estimation proved that myosin ATP-ase activities of cardiac muscle and bone marrow are increased immeadiately after irradiation while the activities of all other excepting these two are considerably reduced. The ATP-ase activities of the liver, spleen, kidney, and Peripheral blood increase gradually 1-3 hours after irradiation while on the contrary the activities of cardiac muscle and bone marrow are found to have decreased markedly 3-6 hours afterwards. Later on the activities of the liver, spleen, kindney and peripheral blood decrease again and after 6-12 hours they show the values below the normal, whereas after 12-24 hours, they recover close to the normal value excepting three cases in perihperal blood. The activities of cardiac mucle and bone marrow decrease for the first 3-6 hours but later increasing gradually, they recover almost to the normal 12-24 hours afterwards. However, after 12 hours the activity of bone marrow declining Gradually, continues its descent without ever recovering. Although the activity of mitochondrial ATP-ase behaves more or less in similar manner as that of myosin ATP-ase, its absolute value is lower than the latter observed at the same time and the former has a tendency to fluctuate prior to the latter. In histochemical observations the intensity of reactions fluctuates rather identically with the fluctuations observed in biochemical study. The reaction of cardiac muscle is markedly reduced for the first three hours after irradiation but it can be somewhat reactivated with cysteine. After six hours the reactions of liver parenchymal cells, biliary tubules, and basillar part of epithelial cells of renal urinary tubules decrease to a certain degree; and this decrease seems to be due to the true ATP-ase activities controlled by the SH group. The reaction of phosphomonoesterase like ATP-ase receiver no conceivable influence with X-ray irradiation. A decrease in the numbers of leukocytic cells is responsible for the striking reduction in the activity of the spleen 12 hours after irradiation. When the liver, kidney and heart in normal conditions are made to react in the absence of substrate, they show extremely slight reaction; whereas after X-ray irradiation, invariably they reveal no such reaction.