The hepatic clearance and drug-drug interactions of luteolin and apigenin were studied by using primary cultured rat hepatocytes. Luteolin and apigenin experienced extensive first-pass metabolism. The elimination percent of luteolin and apigenin was found to be 91.9% and 86.7% after 120 min of incubation. The predicted % liver blood flow was 82.3% and 85.4% for luteolin and apigenin, respectively. Total glucuronidated/sulfated conjugates of luteolin/apigenin were determined by an enzyme hydrolysis method. Compared with the elimination of pure luteolin and apigenin, the elimination of luteolin and apigenin was much lower in hydrolyzed Flos Chrysanthemi extract (FCE) containing comparable amounts of luteolin and apigenin. The effect of a series of flavonoids, including flavonols, flavones, isoflavone, flavanone, flavanonols and catechins, on the elimination of luteolin and apigenin was studied. At least four key determinants in the chemical structures of flavonoids are necessary for exerting the inhibitory effects on the conjugation: 1) catechol structure (3',4'-dihydroxylation) in the B-ring; 2) B-ring is attached to the C-2 position on the C-ring; 3) the C2-3 double bond in conjunction with the C4 carbonyl group on the C-ring; 4) no glycoside present. Investigation of clearance and interaction among flavonoids could help us better understand their bioavailability and offer insight into the approaches to be taken to minimize competitive effects, and to design appropriate bioavailability studies in humans.