Isolation and characterization of the conjugated metabolites excreted in rat bile following the oral administration of a large dose of estriol has been undertaken. The principal conjugates (I-XI) were separated by chromatography on Amberlite XAD-2 resin, followed by gel filtration on Sephadex LH-20 and partition chromatography on silica gel. The structures of these metabolites were deduced from the physico-chemical data and definitely characterized by leading to the known derivatives and their comparison with the synthetic specimens. The physiological significance of biotransformation hereby observed has also been discussed.