Adenosin-triphosphate (ATP) was obtained from the skeletal muscle of rabbits and dogs with Lohmann's method as Ba-salt. Before use this was changed to Na-salt which was proved to be pure chromatographically. Using this Na-salt as substrate observation on ATP ase activity were carried out in tissues of the liver, spleen, heart, kedney, duodenum and skeletal muscles as well as in blood and bone marrow of rabbits. Chemical analysis proved that various organs and tissues gave myosin ATP-ase activity showing the values specific to each organ, i.e. about 10γPi per 100 mg. of wet tissues of heart muscle, liver, spleen, kidney and duodenum, 6γPi per 100 mg. in bone marrow, quite a low value of about 1γPi per 100 mg. in circulating blood. In every organ the activity of mitochondrial ATP-ase was as low as 0.9-10.3γ Pi per 100 mg., quite lower than that of myosin ATP-ase; and as for the differences between the values of myosin and mitochondrial ATP-ases in each organ, it was found that the difference was smallest in peripheral blood and greatest in the heart tissue. As for the practical application in the histochemical study, frozen slices of formalin fixation are thought to be the simplest and best. According to their reactions against activating agents (cysteine) and inhibitory agents (PCMB), ATP-ase acn be divided into four types. The first type is of the typical ATP-ase which is demonstrable only in the frozenslices of cardiac and skeletal muscles, and detected especially in the region of anisotropic disk. This reaction is inhibited by PCMB and reactivated by cysteine. The second type is of those found in frozen slices of basillar part of epithelial cells of renal urinary tubules, liver parenchymal cells and biliary tubules and mooth muscle of duodenel wall. This reaction is inhibited by PCMB but not reactivated by cysteine. The third type is of those appear at the brush border of epithelial cells of renal urinary tubules of paraffin sections. This reaction is markedly inhibited by cysteine but stands unaffected by PCMB. The fourth type is of those detectable in various portions of tissues such as the smooth muscle of arterial media and intima as well as various viscera. The reaction remins unaffected either by cysteine or PCMB in both the frozen and the paraffin sections. The reaction of blood cells is the one that rightly belongs to this type. Reactions observable in paraffin sections are the enzyme actions mainly belonging to the third and the fourth types; and it is believed the reaction due to phosphomoesterase (Ⅲrd tpye) and the unknown enzymes other than true ATP-ase (Ⅳth type). In the case of the slices embedded in methacryleric resin all activities excepting that of the brush border of epithelial cells of renal urinary tubules are lost. The ATP-ase activity of leukocytes is found chiefly in the cytoplasm and it is detected in matured cells to a marked degree.