A method has been developed for simultaneous determination of pool sizes and fractional turnover rates (FTR) of chenodeoxycholic acid (CDCA) and cholic acid (CA) in man by 13C/12C isotope ratio measurements of bile acids in serum after oral administration of 20-50 mg of [24-13C]-labeled bile acids. 13C/12C isotope ratio measurements were performed by capillary gas-liquid chromatography/electron impact mass spectrometry. CA and CDCA kinetics in serum measured by this method exhibited first order kinetics and permitted calculation of pool size and FTR of CA and CDCA. The validity of the measurements in serum was tested by simultaneous measurements in bile in three healthy volunteers and in five patients with various hepatobiliary disorders (three patients with cirrhosis, one with cholecystectomy and sphincterotomy, and one with sphincterotomy only). No consistent differences were found between the pool sizes and FTR's obtained from serum and bile. In a total of five healthy volunteers bile acid kinetics were measured in serum. The values found for the pool sizes and FTR's of CA and CDCA in these subjects were in excellent agreement with data reported in the literature based on 14C or 3H measurements in bile. The pool sizes (mean +/- SD) of CDCA and CA were 32.6 +/- 9.9 and 31.8 +/- 16.0 mumol X kg-1, respectively. The corresponding values for the FTR's were 0.24 +/- 0.13 and 0.48 +/- 0.22 d-1. These data demonstrate that pool sizes and fractional turnover rates of cholic and chenodeoxycholic acid can be measured simultaneously by blood sampling after oral administration of the respective 13C-labeled bile acids.