Ursodeoxycholic acid has been proposed for the treatment of primary biliary cirrhosis. The aim of this study was to evaluate the effect of ursodeoxycholic acid administration on bile acid metabolism in patients with early-stage primary biliary cirrhosis. Biliary bile acid composition, primary bile acid pool sizes, synthesis, and fractional turnover rate were measured before and after four weeks of ursodeoxycholic acid administration (600 mg/day) in nine patients with biopsy-proven primary biliary cirrhosis (stages I-III). Molar percentages of chenodeoxycholic, cholic, and deoxycholic acids in bile were significantly decreased by ursodeoxycholic acid administration, while its biliary concentration increased to 34.2% at the end of the same four-week period. The cholic and chenodeoxycholic acid pools decreased, although not significantly, while the deoxycholic acid pool was reduced by 60% (from 0.7 +/- 0.12 to 0.29 +/- 0.07 mmol, P < 0.002). Primary bile acid synthesis was slightly increased, and fractional turnover rate was significantly increased. The conversion rate of cholic to deoxycholic acid was measured and found to be significantly increased (P < 0.05) after ursodeoxycholic acid administration; however, serum levels of both free and conjugated deoxycholic acid were significantly decreased (from 23.2 +/- 9.7 to 3.8 +/- 1.9 mumol/liter, P < 0.001). We conclude that in patients with primary biliary cirrhosis, ursodeoxycholic acid administration replaces endogenous bile acids in the enterohepatic circulation by increasing bile acid fractional turnover rate without significant increments of their hepatic synthesis.