Bile acid kinetics were measured by isotope dilution, and hourly outputs of bile acid, cholesterol, and phospholipid were quantified by duodenal perfusion over 24 hr including three liquid meals and an overnight fast in 6 gallstone patients during a pretreatment period and two randomized treatment periods with chenodeoxycholic (chenic) acid or cholic acid. During chenic acid ingestion, bile contained predominantly chenyl conjugates. During cholic acid ingestion, bile was composed of about equal amounts of cholyl and deoxycholyl conjugates; chenyl conjugates decreased markedly due in part to a 50% decrease in chenic acid synthesis. Total bile acid pool size doubled in half the patients receiving either bile acid and was not different during treatment with chenic or cholic acid. Compared to cholic acid, chenic acid caused decreased cholesterol output with no difference in bile acid or phospholipid output. Therefore, bile unsaturated with cholesterol entered the duodenum for more hours per day during chenic acid ingestion than during the cholic or pretreatment periods. There was no relationship among bile acid pool size, bile acid output, and hours per day of supersaturated bile; there was an inverse relationship between total pool size and recycling frequency such that bile acid output remained stable over a wide range of pool sizes. Fasting-state gallbladder bile was supersaturated during the cholic and pretreatment periods, but became unsaturated during chenic acid ingestion. However, hours per day of supersaturated bile could not be reliably predicted from the degree of saturation of fasting-state gallbladder bile (r = 0.62). The efficacy of chenic acid and the lack of efficacy of cholic acid for gallstone dissolution appear related to their different specific effects on biliary cholesterol secretion and not to any effect on bile acid and phospholipid secretion or bile acid pool size.