The polyunsaturated fatty acid regulation of lipogenic enzyme gene expression in genetically obese rats (Wistar fatty, non-insulin-dependent diabetes mellitus) has been investigated. The hepatic mRNA concentrations and activities of lipogenic enzymes in the fatty and lean rat were greatly increased by feeding a hydrogenated fat diet to fasted rats, and also reached similar maximum levels with similar time courses. By feeding a corn oil diet, however, the increases were markedly reduced in the lean rats, but were not significantly reduced in the fatty rats. Consequently, when the animals were fed corn oil, the mRNA concentrations and activities in the fatty rats were higher than those in the lean. Thus, it appeared that the higher gene expression in the fatty rats can be ascribed to the defects of polyunsaturated fatty acid suppression. On the other hand, insulin binding to receptors in the liver was reduced by the corn oil diet in the lean rats but was not reduced in the fatty rats (although the insulin binding level was lower in the Wistar fatty rats than in the lean). Changes in the insulin receptor autophosphorylation and kinase activity toward exogenous substrate were similar to the insulin binding. It is suggested that the polyunsaturated fatty acids may not suppress insulin binding activity to receptors in the livers of the fatty rats, probably due to down regulation by hyperinsulinemia. The defects of polyunsaturated fatty acid suppression of lipogenic enzyme gene expression may be one of the factors of obesity.