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Oleuropein, the active constituent of olive leaf extract, possesses anti-oxidant, hypoglycemic, and hypolipidemic activities. We aimed to assess whether the effect of olive leaf extract on hepatic fat accumulation is preventive or therapeutic. Sprague-Dawley (SD) rats were fed a high-fat diet with (ODOD group) or without (HDHD group) olive leaf extract (1,000 mg/kg diet) for 38 weeks. Another group of rats were fed a high-fat diet for 23 weeks, followed by a high-fat diet with olive leaf extract (1,000 mg/kg diet) for 15 weeks (HDOD group). Serology, histopathology, anti-oxidative activity, and liver fatty acid synthesis were compared to those fed a standard diet (LDLD group) at 26 and 41 weeks of age. The serum levels of total cholesterol, triglyceride and aspartate aminotransferase tended to be lower in the ODOD group as compared to the HDHD and HDOD groups, although there were no significant differences. Histopathologically, hepatic steatosis tended to be less evident in the HDOD and ODOD groups as compared to the HDHD group, and lobular inflammation was not observed in the ODOD group at 26 weeks of age. Hepatic thioredoxin-1 staining tended to be less evident in the ODOD group than in the HDHD and HDOD groups at 41 weeks of age. There were no significant differences in hepatic lipogenic enzyme activities between the ODOD group and HDHD/HDOD groups. Our data suggest that olive leaf extract had a preventive, rather than therapeutic, effect on hepatic steatohepatitis in SD rats fed a high-fat diet., Acta Medica Nagasakiensia, 55(1), pp.29-39; 2010