The aims of this proposal are to investigate the mechanism responsible for observed differences in bile acid pool size measurements between serum and bile, to validate a single sample serum technique using two stable isotopes for measurement of bile acid kinetics, to investigate the effects of pregnancy and oral contraceptive steroids on conversion of the bile acid intermediate, 7 Alpha-hydroxy-4-cholesten-3-one, to chenodeoxycholic acid (CDCA) and cholic acid (CA) and to directly measure bile acid synthesis using an 18O inhalation technique. All of the following experiments will be performed using gas chromatography/mass spectromatry/stable isotope ratiometry. Biliary and serum bile acid kinetics will be compared in 5 subjects with gallbladders and 5 subjects post-cholecystectomy using 13C-24-CDCA and 13C-24-CA. In 10 subjects we will compare serum bile acid kinetics obtained from one serum sample by double isotope dilution technique with those using the multiple sample serum technique. In 5 pregnant women and 5 women taking contraceptive steroids we will study the conversion of a key bile acid intermediate, 7Alpha-hydroxy-4-cholesten-3-one (HCO) to CDCA and CA, using deuterated HCO and 13C-24-CDCA, and 13-24-CA. The fraction of HCO metabolized to CDCA AND CA will be compared to the production rates of CDCA and CA. We will directly measure bile acid synthesis in 5 healthy subjects on and off cholestyramine by measuring the incorporation of 180 into bile acid hydroxyl groups after inhalation of an 180 enriched atmosphere. These studies will explain the difference we have observed between pool size measured from serum and bile. They will greatly expand our ability to do ble acid kinetics because of improved subject compliance and decreased analytical time in the mass spectrometry laboratory. The HCO study may pinpoint the pathway of bile acid biosynthesis that is altered by female steroid hormones and thereby provide indirect information about the intracellular localization of action of female steroid hormones. This information will aid in understanding the pathogenesis of the increased risk of cholesterol gallstones in women especially those exposed to high concentrations of female steroid hormones. The 180 study will validate a technique for subsequent measurement of the short-term effect of female steroid hormones on bile acid synthesis.