The LEC rat, which suffers from hereditary hepatitis, was examined for elucidation of its clinicopathological characteristics during development of the acute phase of hepatitis by quantitative analyses of histological observations of the liver in combination with laboratory data on various serum enzymes. The progression of acute hepatitis in the LEC rat was observed to begin insidiously early in life, i.e., a few enlarged hepatocytes and Councilman bodies appeared at around 8 weeks of age without clinical signs. Furthermore, it was revealed that the acute phase of hepatitis started with a remarkable increase of Councilman bodies, large nuclei and hepatocytes in mitosis in the liver 3 to 4 weeks before the onset of fulminant hepatitis, which is characterized by the elevation of serum enzyme activities such as GOT, GPT and gamma-GTP, and the onset of jaundice. From those observations, three stages were proposed for the progression of acute hepatitis in the LEC rat.