p59fyn, a protein tyrosine kinase belonging to the src-family, is involved in the regulatory mechanism of acute response to ethanol in the central nervous system. A previous report showed an association between src-family kinase activity and fatty acid oxidation, and it also reported that hepatic free fatty acid levels were low in Fyn−/− mice. We examined, using Fyn−/− mice whether Fyn is also involved in fatty acid metabolism and the development of pathological changes in the liver in response to chronic ethanol consumption. C57BL/6J Fyn−/− and Fyn+/+ mice were fed for 8 weeks with either a liquid diet comprising ethanol or one in which the calories from ethanol were replaced with carbohydrates. Chronic ethanol consumption for 8 weeks resulted in remarkable hepatic steatosis in Fyn+/+ mice but not in Fyn−/− mice. Chronic ethanol consumption induced a significant decrease in hepatic FFA and triglyceride levels in Fyn−/− mice. Levels of interleukin-6, which is associated with the enhancement of fatty acid oxidation, was also increased significantly in the livers of ethanol-fed Fyn−/− mice. The results suggest that Fyn is involved in the enhancement of fatty acid oxidation and the development of hepatic steatosis caused by chronic ethanol consumption.