The effect of exogenous fatty acids on expression of the stearoyl-CoA desaturase-1 (SCD1) gene was assessed both in vivo and ex vivo. Mice fed a fat-free diet or a diet containing a largely monounsaturated (18:1) fat, i.e., olive oil, expressed high levels of hepatic SCD1 mRNA. In contrast, in mice fed diets containing primarily polyunsaturated (18:2 and 18:3) fats, expression of the hepatic SCD1 message was markedly suppressed. Similar experiments with pure fatty acid esters showed that arachidonate (20:4) and linoleate (18:2) were far more potent in down-regulating expression of the hepatic SCD1 message than oleate (18:1), an end-product (as its CoA thioester) of the SCD1-catalyzed reaction. The reduction of hepatic SCD1 mRNA appears to be primarily due to inhibition of SCD1 gene transcription since polyunsaturated fatty acids caused a decrease in run-on transcription of the gene comparable to the decrease in message level. Consistent with the effects observed in vivo, unsaturated fatty acids suppressed the expression of SCD1 mRNA by rat hepatocytes cultured in serum-free medium. Suppression increased with degree of unsaturation with arachidonic (20:4) and eicosapentaenoic (20:5) acids, causing a > or = 90% reduction in the level of SCD1 message. Thus, the SCD1 gene, like the fatty acid synthase and S14 genes, undergoes coordinate transcriptional down-regulation in response to unsaturated fatty acids.