Among the various anti-leukemic agents, nitrogen mustrad and its derivatives have the similar biological effects identical with x-ray inactivating the SH group. Following up the author's former reports, in which a marked fluctuation of ATP-ase activity in various tissues of the x-ray irradiated mice was described, the author reports the effect of NMNO on the ATP-ase activities of the liver, spleen, heart, kidney and duodenum of mice. By a single injection of 1% NMNO, 300 mg/Kg., most of the adult mice died after 6 to 8hours. Hourly observations revealed the marked fluctuation of ATP-ase activitity in various organs as in the case of x-ray irradiation. The activities both of myosin and mitochondrial ATP-ase decreased slightly in every organ 30 minutes after the injection of NMNO, and then increase slightly reaching nearly the normal level after 3 hours. Further obverations revealed that the transient recovery of activities decreased again, reaching the level of inorganic P contents proper to each organ 7 hours after injection. The histochemical observation revealed also a marked decrease in the activity of ATP-ase. The inhibitory effect of NMNO to the ATP-ase activities was also proved in vitro, adding 10(-2)mol. NMNO in the incubating media. The colchine, a mitotic poison, added in the same media 10(-2)mol. effected only a slight decrease in ATP-ase activity. The incubation with L-cysteine after the treatment wiht NMNO restored the ATP-ase activity to a certain degrree. In the case treated with NMNO a marked decrease in the reactions of liver parenchymal cells, biliary tubules, blood cells of splenic pulp, cardiac fibers, the basillar part of epithelial cells of renal tubules is observable; and reactions of peripheral blood and blood in bone marrow, which received no effect from X-ray irradiation, are reduced markedly. However, when the reaction is made to take place in the substrate loaded with cysteine, only the reactions of cardiac fibers and liver parenchymal cells are restored to a certain degree.