Technical Abstract: Citrus flavonoids have been shown to decrease plasma lipid levels, improve glucose tolerance, and attenuate obesity. One possible mechanism underlying these physiological effects is reduction of hepatic levels of the mRNA for stearoyl CoA desaturase-1 (SCD1), since repression of this enzyme reduces hyperlipidemia and adiposity. Here, we show that citrus flavonoids of two structural classes reduce SCD1 mRNA concentrations in a dose-dependent manner in rat primary hepatocytes. This is the first demonstration of repression of SCD1 by citrus flavonoids, either in vivo or in cultured cells. Furthermore, it is the first use of freshly-isolated hepatocytes from any animal to examine citrus flavonoid action at the mRNA level. This study demonstrates that regulation of SCD1 gene expression may play a role in control of obesity by citrus flavonoids and that rat primary hepatocytes are a physiologically-relevant model system for analyzing the molecular mechanisms of flavonoid action in the liver.