Cholesterol 7α-hydroxylase, the rate-limiting enzyme in the bile acid synthesis pathway, is downregulated by taurocholate by way of negative feedback control at the level of gene transcription. The molecular basis of regulation of cholesterol 7α-hydroxylase by other hydrophobic bile salts and under more physiological conditions is not known. The aim of this study was to investigate the molecular basis of regulation of cholesterol 7α-hydroxylase by several naturally occurring bile salts in rats with intact enterohepatic circulation. Male Sprague-Dawley rats were pair-fed for 14 days normal chow (control), cholestyramine (5% of diet), cholic acid (1%), chenodeoxycholic acid (1%) or deoxycholic acid (0.25%). When rats were killed, livers were harvested and HMG–CoA reductase specific activity and cholesterol 7α-hydroxylase specific activities, steady-state mRNA levels and transcriptional activity were determined Nuovo Cimento with those of control rats fed normal chow. Compared with results in paired controls, cholestyramine feeding led to an approximate threefold increase in HMG-CoA reductase specific activity. Feeding of hydrophobic bile salts profoundly decreased the specific activity of HMG-CoA reductase. Cholestyramine led to a three-fold increase in cholesterol 7α-hydroxylase specific activity, steady-state mRNA levels and gene transcriptional activity. The feeding of cholic (1%), chenodeoxycholic (1%) and deoxycholic acid (0.25%) led to significant decreases in cholesterol 7α-hydroxylase specific activities (62%, 84% and 97%, respectively), steady-state mRNA levels (72%, 29% and 61%, respectively) and transcriptional activities (44%, 43% and 54%, respectively). Down-regulation of cholesterol 7α-hydroxylase specific activity was in order of increasing hydrophobicity of bile salts (cholic < chenodeoxycholic < deoxycholic acid). No such clear correlation was observed between bile salt hydrophobicity and steadystate mRNA levels or gene transcriptional activity. We conclude that down-regulation of cholesterol 7α-hydroxylase activity by cholic, chenodeoxycholic and deoxycholic acids occurred as the result of decreased transcriptional activity of the cholesterol 7α-hydroxylase gene. Because chenodeoxycholic and deoxycholic acids led to greater fractional suppression of cholesterol 7α-hydroxylase specific activity than of gene transcriptional activity, we postulate the existence of posttranscriptional regulation of cholesterol 7α-hydroxylase by these two hydrophobic bile salts. (HEPATOLOGY 1994;19:941–947.)