In 38 healthy human milk fed preterm infants (gestational age 27 - 30 weeks (n = 10); 31 - 32 weeks (n = 21); 33 - 34 weeks (n=7)) the preprandial total bile acid (BA) concentrations in serum and duodenal juice were simultaneausly measured during the first 60 days of life. The BA were enzymatically determined using 3-α-hydroxysteroid dehydrogenase. In the duodenal juice, the cholic acid (CA), chenodeoxycholic acid (CDCA), deoxycholic acid and lithocholic acid were separately quantified by thinlayer chromatography. During the first month of life the serum BA concentrations increased significantly with postnatal age (p < 0.01) but remained nearly constant during the second month of life. In the duodenal aspirates, the BA concentrations increased continuously up to the end of the observation period (p < 0.001). In the duodenal aspirates, the CA/CDCA ratio was high immediately after birth and decreased significantly with increasing postnatal age (p < 0.001). During the first weeks of life, the BA were preferentially conjugated with taurine, but taurine/glycine ratio decreased with postnatal age (p < 0.001). In all samples of duodenal juice the sum of primary BA was > 98% of total 3-α-hydroxy-BA. There were no significant differences between the gestational age related groups for all studied parameter. The data indicate that the BA transport from portal blood to the duodenum is the limiting step of the enterohepatric bile acid circulation in preterm infants during the first weeks of life. Duodenal BA concentrations below 4 mmol/l, as found in this study, may be a limiting factor for fat absorption. The data also indicate that the intestinal flora necessary for bile acid conversion has not been established during the first 8 weeks of life.