Studied were the morphologic changes in the organs of 17 calves for fattening with massive acute and subacute acidosis of the rumen. The disease broke out as the result of an abrupt change in the regime of feeding and tending. The gross observations consisted of hemorrhagic and necrotic foci in the rumen, hemorrhages and hyperemia in the omasum and reticulum, and ulcera in the abomasum. Established was also enlargement of the lymph nodes in the forestomach as well as odemas in the liver and kidneys. Histologically, there were necrotic inflammation of the rumen, serous and fibrinous exudate, leukocyte infiltration, hyphae, thrombuses, and arteritis. The omasum and reticulum presented desquamation of the cutaneous mucosal membrane, and the abomasum--necrotic foci, edema, and leukocyte infiltrations. There was also hepatitis, and fatty infiltration of the kidneys and heart and an activation of the interstitium. Degenerative process and hyperemia were observed in the thyroid gland, and fatty infiltration--in the pancreas. The kidneys, the adrenal glands, and the liver had higher alkalinephosphatase activity.