Glucuronic acid conjugates of several foreign compounds (1- and 2-naphthols, 2-, 3-, and 4-hydroxybiphenyls, m- and p-hydroxyphenylphenylhydantoins) were produced in the isolated perfused rat liver and identified in the bile as the permethyl derivatives. Permethylated glucuronide isomers were easily separated by gas chromatography on SE-30 and OV-17 columns. The mass spectra of permethylated glucuronide isomers were very similar. Gas chromatography proved to be most useful for the separation and identification of isomeric glucuronides.