Administration by duodenal intubation of 25 mg of oestriol to a patient with a complete biliary fistula resulted in the gradual disappearance of free oestriol from the duodenal contents and in the appearance of increasing amounts of conjugated oestriol. When sodium oestriol-16(17?)-glucosiduronate was given in a similar fashion, only conjugated forms were found. Injection of 50 mg amounts of oestriol into an isolated jejunal loop with a preserved arterial blood supply gave rise to huge quantities of conjugated oestriol in the intestinal wall as well as in the effluent venous blood. Smaller, but still significant amounts of conjugates were formed following intra-gastric oestriol administration, whereas very slight, or no conjugating capacity could be demonstrated in an experiment involving oestriol injection into the large intestine. Sodium oestriol-16(17?)-glucosiduronate has been isolated and identified in extracts of the intestinal wall and effluent venous blood. Another oestriol glucosiduronate (probably a di- or triglucosiduronate) has also been detected. Very little, if any, sodium oestriol-3-sulphate was found. It is concluded that certain parts of the human gastro-intestinal tract possess the ability to convert huge amounts of oestriol into oestriol glucosiduronates. It is suggested that other steroids may also undergo intestinal conjugation.