In this study, the differences between the pattern of stained proteins from genetically obese Zucker rats and those from lean Zucker rats were analyzed using SDS-polyacrylamide gel electrophoresis. The level of the 120 kDa protein in the liver cytosol fraction of the obese rats was several times higher than that found in the lean rats. This protein was not present in the mitochondrial fractions of either strain. The level of the 120 kDa protein was decreased drastically during a 72-h fast in the obese Zucker rats. An increase in the level of this protein was induced in the lean Zucker rats through a 48-h fast followed by refeeding with a high-carbohydrate diet. However, when the lean Zucker rats were refed with a high-fat diet following the fast, no significant change was observed in the level of the 120 kDa protein. These observations strongly suggest that the elevated levels of the 120 kDa protein seen in the liver cytosol fraction of obese Zucker rats may be responsible for the observed in-creases in lipogenesis and pathogenesis of obesity in these rats.