The inducing effects of some flavonoids (flavone, flavanone, tangeretin and quercetin) and model substances have been studied in rats, and the activity and the expression of drug-metabolizing enzymes have been compared in rats. The addition of flavonoids to the diet (0.3% w/w) for 2 weeks did not change the liver cytochrome P450 content nor the activities of the NADPH-cytochrome P450 and NADH-cytochrome b5 reductases, but it affected the activities of phase I and phase II enzymes. Flavone, and to a lesser extent tangeretin, increased the activities mediated by the P450 1A1,2 (EROD) and 2B1,2 (PROD) as well as the activities of p-nitrophenol UDP-glucuronyl transferase (UGT) and glutathione transferase (GST). Flavanone mainly enhanced PROD, UGT and GST, whereas quercetin did not modify any enzyme activities. None of the tested flavonoids modulated the activities catalyzed by P450 2E1, 3A and 4A. Immunoblotting studies showed that flavone and tangeretin increased the expression of cytochrome P450 1A and 2B forms, whereas flavanone only induced cytochrome P450 2B. Flavone and to a lesser extent flavanone, markedly increased the phenol-UGT protein level. Both flavone and flavanone also increased the androsterone- and testosterone-UGTs, whereas tangeretin and quercetin did not increase any UGT isoform. We concluded that the flavonoids tested specifically affected the expression of the drug-metabolizing isozymes in rat liver, their inducing properties were dependent on their chemical structures.