In order to clarify the histopathological changes resulting from nitrate poisoning, sheep were experimentally poisoned by the ruminal administration of 0.56g sodium nitrate per ㎏ body weight. Microscopic findings in lung, heart, liver and kideny were studied. The results obtained were summarized as follows: 1. Main findings observed from the acute cases which died after the administration were severe cyanosis of visible mucosa, frequent urination, and dyspnea. 2. Main gross appearances observed in acute cases were dark red discoloration of blood, syanosis of mucosa, and hyperemia of internal organs. 3. Main microscopic appearances observed in acute cases were hemorrhage and congestion of various organs, centrilobular necrosis of hepatic parenchyma, and necrosis of renal convoluted tubules.