We have previously reported that relatively hydrophobic bile acids, decreased hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase (reductase) activity whereas, hydrophilic bile acids had little effect on the enzyme. The purpose of the present study was to determine in more detail the mechanism of down-regulation of hepatic reductase activity by hydrophobic bile salts. Groups of rats were fed bile acids of differing hydrophobicity: ursodeoxycholic, cholic (CA), chenodeoxycholic (CDCA), deoxycholic (DCA), or cholesterol for 14 days. Reductase specific activities and concentrations of reductase protein were determined in hepatic microsomes. Quantitation of "steady state" levels of reductase mRNA was performed using Northern and dot blot hybridization. Reductase gene transcriptional activity (nuclear "run-on") was determined in nuclei isolated from livers of animals fed different bile acids. Hydrophobic bile acids and cholesterol significantly decreased reductase activity: CA (57%), CDCA (77%), DCA (73%), cholesterol (89%), and reductase protein levels as measured by an enzyme-linked immunosorbent assay method were also decreased; CA (27%), CDCA (31%), DCA (42%), and cholesterol (35%). Reductase mRNA levels were also decreased after feeding hydrophobic bile acid: CA (43%), CDCA (47%), DCA (54%), and cholesterol (53%). Ursodeoxycholic, a hydrophilic bile acid, caused a much smaller decrease in reductase activity (18%), protein mass (16%), and mRNA levels (10%). Decreased transcriptional activities were observed in CA- and cholesterol-fed rats. Surprisingly, CDCA- and DCA-fed animals showed transcriptional activities similar to control animals even though steady state mRNA levels were low in CDCA- and DCA-fed animals. We hypothesize a post-transcriptional regulation of reductase mRNA by hydrophobic bile acids.