(R)-(+)-Menthofuran is a metabolite of (R)-(+)-pulegone, the chief constituent of pennyroyal oil. Menthofuran has been shown to account for a significant percentage of pulegone toxicity through further metabolism to a reactive intermediate, an enonal (2-Z-(2'-keto-4'-methylcyclohexylidene)propanal). Hydration of the enonal followed by a 1,4-dehydration and rearrangement gives rise to diastereomeric (-)-mintlactone and (+)-isomintlactone (mintlactones). We have conducted disposition studies on pulegone as part of the National Toxicology Program initiative in herbal medicines and dietary supplements, and have reported previously unknown urinary metabolites of pulegone. Comparative metabolism studies of 14C-labeled menthofuran in Fischer-344 (F344) rats were carried out to determine urinary metabolites of pulegone that are derived from the menthofuran pathway. Three sulfonic acid metabolites, namely, hexahydro-3,6-dimethyl-1-(2-sulfoethyl)-2H-indol-2-one, hexahydro-3,6-dimethyl-7a-sulfo-2(3H)-benzofuranone, and 2-sulfomenthofuran, were identified in urine of treated rats. Formation of these metabolites may be derived from reactions of the enonal with taurine or glutathione (GSH) (or sulfite ion). Other identified urinary metabolites of menthofuran could be attributed to further metabolism of mintlactones. Further hydroxylation of mintlactones could give 7a-hydroxymintlactone and 6,7a-dihydroxymintlactone. Glucuronidation or reduction of 7a-hydroxymintlactone could give rise to the major metabolites 7a-hydroxymintlactone glucuronide and 2-[2'-keto-4'-methylcyclohexyl]propionic acids. Glucuronidation or repeated hydroxylation/dehydration of 2-[2'-keto-4'-methylcyclohexyl]propionic acids could result in formation of hexahydro-3,6-dimethyl-7a-hydroxy-2(3H)-benzofuranone glucuronide and 2-(2'-hydroxy-4'-methylphenyl)propionic acid. 2-(Glutathion-S-yl)menthofuran, a GSH conjugate of the enonal that has been partially characterized in bile of rats dosed with pulegone, is at most a minor biliary metabolite of menthofuran in rats.