Simultaneous umbilical cord arterial, cord venous, and maternal peripheral venous blood samples were collected in labour in twenty-five uncomplicated human pregnancies at term, and the concentrations of serum bile acids were determined by a gas chromatographic method. Cholic acid (C) predominated in the maternal and chenodeoxycholic acid (CD) in the fetal serum samples; the mean ratios of C to CD were 1.50 and 0.92, respectively. Mean concentrations of C and CD were significantly higher in the cord venous (1.27 and 1.55 mumol/l, respectively) than in the maternal venous (0.81 and 0.63 mumol/l, respectively) serum samples. Paired cord arteriovenous comparisons showed that concentrations of C and CD were slightly higher in the cord arterial as compared with the cord venous serum. These results indicate that the placenta is capable to maintain a considerable concentration difference of bile salt between the fetal and maternal compartment. It seems on the basis of cord arteriovenous differences that C and CD are to some extent eliminated from the fetal to the maternal compartment by transplacental passage.