Many furan-containing compounds have been reported to be cytotoxic and/or carcinogenic agents. The toxic furans exert their adverse effects possibly through metabolic activation of the furans to corresponding epoxides or/and cis-enediones. Detection of the reactive metabolites is a challenge since the electrophiles often have short lives in vivo, and they are too reactive to be isolated for characterization. Seven compounds, including 2,5-dimethylfuran, R-(+)-menthofuran, R-(+)-pulegone, caesalmin C, furanodiene, diosbulbin B, and limonin, were selected for a biomarker search study. Glutathione (GSH) conjugates derived from the test compounds were detected in bile of rats, and the types of the biliary GSH conjugates observed differed from each other and were unpredictable. However, upon mixing of the bile with a solution of N-acetyl lysine (NAL), pyrroles derived from NAL and GSH were exclusively detected in all the bile samples without any exception. The formation of the pyrrole-NAL/GSH conjugates was verified by microsomal incubations and chemical synthesis. The findings facilitate the development of in vivo biomarkers of metabolic activation of furanoids.