1. (R)-(+)-Pulegone is a monoterpene that is oxidized by cytochromes P-450 to reactive metabolites that initiate events in the pathogenesis of hepatotoxicity in mice, rats and humans. 2. Selective labelling of (R)-(+)-pulegone with deuterium revealed that menthofuran was a proximate hepatotoxic metabolite formed by oxidation of the allylic methyl groups of pulegone. Incubations of pulegone with mouse liver microsomes in an atmosphere of 18O2 resulted in the formation of menthofuran that contained only oxygen-18 in the furan moiety. These results are consistent with oxidation of pulegone to an allylic alcohol that reacts intramolecularly with the ketone moiety to form a hemiketal that subsequently dehydrates to generate menthofuran. 3. Studies on the metabolism of menthofuran revealed that it is oxidized by cytochromes P-450 to an electrophilic gamma-ketoenal that reacts with nucleophilic groups on proteins to form covalent adducts. In addition, diastereomeric mintlactones are formed. Investigations with H2(18)O and 18O2 are indicative of a furan epoxide intermediate, or a precursor, in the formation of the gamma-ketoenal and mintlactones.