Bile acids are synthesized in the liver from cholesterol by the action of hepatic enzymes and excreted into the small intestine via the bile duct. In the intestinal lumen, they assist lipolysis and the absorption of fats by forming mixed micelles, and then return to the liver upon absorption in the ileum and proximal colon. Because of their efficient hepatic uptake, bile acids have low concentrations in the peripheral blood. Recent observations also indicate that the nuclear bile acid receptor regulates the bile acid pool by repressing transcription of genes encoding hepatocyte transporters[1] as well as cholesterol 7α-hydroxylase[2,3], which is the rate-limiting enzyme for bile acid biosynthesis.