Three metabolites of mebendazole were isolated from the bile of rats dosed with a mixture of mebendazole and pentadeuteromebendazole. The identification was based upon the appearance of the characteristic doublet in the mass spectrum of the compounds and the comparison of their fragmentations with those of authentic compounds. Cochromatography of the metabolites with the authentic compounds on HPLC supported the identification. Methyl-5(6)-(alpha-hydroxybenzyl)-2-benzimidazole carbamate, 2-amino-5(6)-(alpha-hydroxybenzyl) benzimidazole and 2-amino-5(6)-benzoylbenzimidazole were identified as metabolites after enzymic conjugate hydrolysis. Some unmetabolized mebendazole was also found.