Nonalcoholic fatty liver disease (NAFLD) is a common disease in humans having a broad spectrum of liver histology from simple fatty liver to mixed inflammatory cell infiltration and fibrosis (nonalcoholic steatohepatitis, NASH), which is a more severe and progressing form. NASH/NAFLD is significantly associated with lifestyle such as diet and exercise, obesity, insulin resistance, type 2 diabetes, dyslipidemia and hypertension. Age and gender are also associated with the development. On the other hand, NAFLD has been found in a high percentage of nonobese individuals in the Asia-Pacific area. Some characteristic animal models of NAFLD/NASH have been developed to clarify the pathogenesis of human NAFLD/NASH. We have recently developed a novel NASH rat model (stroke-prone spontaneously hypertensive rats, SHRSP5/Dmcr), which showed hepatic steatosis and inflammation at 2 weeks, ballooning, macrovesicular steatosis and fibrosis at 8 weeks, and bridging fibrosis at 14 weeks by feeding of high-fat and -cholesterol (HFC) diet alone. This animal model does not have obesity, insulin resistance or diabetes. Therefore, this may be an excellent animal model of human NASH/NAFLD without obesity and diabetes. Sex and strain differences observed in fibrogenesis by the HFC diet in SHRSP5/Dmcr may be associated with the sensitivity to detoxification enzymes in the liver, because the levels of UGP-glucuronosyltransferase and sulfotransferase and their regulating nuclear receptors only decreased in male SHRSP5/Dmcr rats, but not in female and SHRSP rats. This suggests the importance of phase II reactions of drug-metabolizing enzymes in NASH progression. Importantly, SHRSP5/Dmcr rats are spontaneously hypertensive; therefore, when we use the original strain Wistar Kyoto, which has normal blood pressure, the involvement of blood pressure in the development of human NASH/NAFLD may also be clarified.