Adenosine triphosphatase (ATPase) was visualized by a histochemical technique developed by Wachstein & Meisel. The liver biopsy specimens of 112 cases of various liver diseases served as the materials. The results were as follows: 1) Magnesium activated ATPase distributed chiefly in the perisinusoidal lining cells and biliary capillary in the normal liver. 2) The activity of perisinusoidal lining cells was moderately increased in acute stage of viral hepatitis, notably in perilobular region. The finding was manifest in icteric cases better than anicteric cases. The activity of biliary capillary also increased, again notably in icteric cases. The activity was accentuated at the site of bile thrombi. A variety of unusual morphological changes of biliary trees were postulated from the histochemical visualization of ATPase along biliary capillary. Snaky and cystic appearance, and focal dilatation of the capillary were often encountered. 3) In chronic stage, the enzyme activity of both biliary capillary and perisinusoidal wall were intense in active chronic hepatitis, whereas not so intense in inactive chronic hepatitis. 4) In precirrhotic stage, the enzyme activity was intense in portal triads and the vicinity of progressive fibrotic process took place. 5) In cirrhotic stage, the activity of perisinusoidal wall was less intense in the regenerative nodule in comparison with internodular areas. In contrast the activity of biliary capillary was intense in the regenerative nodule in comparison with internodular areas. Furthermore, a variety of the activity was noted among the regenerative nodules. 6) Generally speaking, the activity was less intense in the fatty liver. The morphological changes of biliary capillary assess fatty cells or cysts. 7) An increased activity of perisinusoidal lining cells was observed in hemolytic janudice. Whereas a decreased activity of biliary capillary in the centrologular areas was noticed in Dubin-Johnson's syndrome. 8) The increased activity of ATPase in biliary capillary was thought to suggest stagnation of bile and subsequent mechanical change of biliary trees. While the increased ATPase activity in perisinusoidal wall was thought to be related to collagen formation since the site of intense activity was consistent to the site of progressive proliferation of collagen fiber.