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The effects of thyme (Thymus vulgaris L.) leaves and its phenolic compounds, thymol and carvacrol, on the activities of xenobiotic-metabolizing enzymes, i.e., phase I enzymes such as 7-ethoxycoumarin O-deethylase (ECOD) and phase II enzymes such as glutathione S-transferase (GST) and quinone reductase (QR), were investigated. Mice were fed with a diet containing thyme (0.5% or 2.0%) or treated orally with thymol (50–200 mg/kg) or carvacrol (50–200 mg/kg) once a day for 7 successive days, and then the enzyme activities in the livers were analyzed. Dietary administration of 2% thyme caused slightly but significantly higher ECOD, GST, and QR activities by 1.1–1.4-fold. Thymol (200 mg/kg) treatment resulted in significantly higher ECOD, GST, and QR activities by 1.3–1.9-fold, and carvacrol (200 mg/kg) treatment caused significantly higher ECOD, GST, and QR activities by 1.3–1.7-fold. Thymol-treated animals had significantly higher protein levels of GST α and GST µ, and carvacrol-treated animals had significantly higher levels of GST µ. These results imply that thyme contains bifunctional inducers (i.e., substances capable of inducing both phase I and phase II enzymes) and that thymol and carvacrol may account for the effects of thyme.