Cholesterol, along with a number of its derivatives and very small quantities of its precursors, is the only sterol which can be identified in extracts of mammalian tissue. Cholesterol is eliminated from the body in the form of bile acids, the degradation products of the steroid hormones and by way of excretion of neutral sterols in the faeces. The formation of the bile acids is the major quantitative pathway, . In the rat, the major bile acids which have been identified in bile are the dihydroxy acid, chenodeoxycholic acid and the trihydroxy acid, cholic acid. Small quantities of the muricholic acids (trihydroxy acids) have also been identified. There is no interconversion of chenodeoxycholic acid and cholic acid in the rat. The bile acids are excreted in bile as conjugates, mainly with taurine and are reabsorbed from the small intestine and returned to the liver in the portal blood. The re-excretion of the bile acid conjugates in bile thus establishes an enterohepatic circulation. The total biliary content of bile acid conjugates represents both those conjugates which have been reabsorbed from the small intestine and a small quantity which have been newly synthesised. Thus direct measurement of the concentration of bile acid conjugates does not provide an estimate of the bile acid synthesis rates.