This investigation aims to explore the metabolism of ursodeoxycholic acid in human subjects with gallstones treated either with ursodeoxycholic acid or chenodeoxycholic acid. Specifically, we will conduct a clinical trial to determine if ursodeoxycholic acid is a superior litholytic (gallstone dissolving) agent than chenodeoxycholic acid and if fewer side effects are noted during treatment with ursodeoxycholic acid. Other parameters that will be measured during this trial include (a) pool size and daily production rate by isotope dilution techniques (b) effect on hepatic microsomal HMG CoA reductase and cholesterol 7 alpha-hydroxylase activity (c) difference in the bacterial metabolism of the 7 alpha and 7 beta hydroxyl groups of chenodeoxycholic acid and ursodeoxycholic acid (d) effect on intestinal cholesterol absorption (e) pathway and tissue site of formation of ursodeoxycholic acid.