Mitochondrial ATPase was studied in mouse tissues. The ATPase activity was varied in the tissues studied. The protein concentration of mitochondria was decreased in the order of liver, kidney, testis, lung, spleen, heart, brain and muscle in young mouse, and in the order of liver, testis, lung, kidney, heart, spleen, brain and muscle in old mouse. The mitochondrial ATPase activity was decreased in the order of muscle, lung, heart, brain, kidney, spleen, muscle, liver and testis in young mouse and brain, heart, lung, kidney, spleen, muscle, liver and testis in old mouse. Effect of inhibition by DCCD was decreased in the order of spleen, lung, liver, testis, heart, muscle, kidney and brain in young mouse and spleen, lung, heart, muscle, testis, kidney, liver and brain in old mouse. Effect of activation by DNP was decreased in the order of kidney, spleen, muscle, liver, heart, testis, lung and brain in young mouse and lung, muscle, spleen, liver, heart, kidney, brain and testis in old mouse. The electrophoretic mobility of mitochondrial ATPase was not identical in young mouse tissues: the electrophoretic mobility of muscle enzyme was fast. That of kidney, heart, liver and spleen was slow and that of lung, testis and brain was middle. The electrophoretic mobility of enzyme of old mouse tissues was similar.