1. The influence of the dietary flavonoids, chrysin, quercetin, tangeretin, flavone and flavanone, on the components of the rat liver drug-metabolizing enzyme system was examined and compared with two well-known synthetic flavonoids 7,8-benzoflavone and 5,6-benzoflavone. 2. Polyhydroxylated molecules such as quercetin and chrysin, produced no significant changes on phase I and phase II enzyme activities. 3. Flavone was the most potent inducer, and resulted in a mixed type of induction. 7-Ethoxycoumarin O-deethylase (ECOD), 7-ethoxyresorufin O-deethylase (EROD) and pentoxyresorufin depentylase (PROD) activities were increased 2, 30 and 15-fold respectively. p-Nitrophenol UDP-glucuronyltransferase (UDPGT 1), p-hydroxybiphenyl UDP-glucuronyltransferase (UDPGT 2) and glutathione transferase (GST) activities were also induced. 4. Flavanone, which differs from flavone only by the degree of unsaturation of the pyrone ring, produced only a weak increase in monooxygenase activity, but the increase in phase II enzyme activities was as great as that for flavone treatment. 5. Tangeretin displayed a mixed pattern of induction, with increases in ECOD, EROD and PROD, and UDPGT 1 and UDPGT 2 activities, but these were less than with flavone treatment. 6. 7,8-Benzoflavone and 5,6-benzoflavone showed induction patterns similar to those of 3-methylcholanthrene. Nevertheless dietary treatment with 5,6-benzoflavone caused changes which were not as great as those usually described when this compound is administered i.p.