Discussion and SummaryCholic acid‐24–14C was administered intraperitoneally to rats which had been given terramycin and sulftalyl per os twice daily for three days before and during the whole experiment. Chromatographic separation of the labelled bile acid products excreted in faeces showed that the major part (75–95 per cent) is excreted as taurocholic acid. 2–10 per cent is presumably excreted as glycocholic acid. Likewise in the bile the labelled cholic acid has been found mainly in the form of taurocholic acid, only a minor part being present as glycocholic acid.About 1–15 per cent of the labelled compound is found at the place where free cholic acid occurred. None of the bile acid metabolites found in normal rat faeces is observed after administration of antibiotics. This indicates that most of the peptide bonds of the conjugated bile acids are split by the intestinal microorganisms and that these latter also cause modification of the bile acid molecules. Further work is needed to decide whether the small amounts of free bile acid found in these experiments are due to splitting by resistant microorganisms, fungi or intestinal digestive enzymes.