The hypouricemic actions of Biota orientalis (BO) extract and its flavonoid constituents quercetin and rutin, were in vivo examined using oxonate-induced hyperuricemic mice. Quercetin and rutin, when administered three times orally to the oxonate-induced hyperuricemic mice, were able to elicit dose-dependent hypouricemic effects. The effects of quercetin and rutin were more potent than that of Biota orientalis extract at the same dose of 100 mg/kg. At doses of 50 mg/kg of quercetin or above, or at doses of 100 mg/kg of rutin or above, the serum urate levels of the oxonate-pretreated mice were not different from normal mice. In addition, Biota orientalis extract, quercetin and rutin, when tested in vivo on mouse liver homogenates, elicited significant inhibitory actions on the xanthine dehydrogenase/xanthine oxidase (XDH/XO) activities. The effects of quercetin and rutin resulted less potent than that of allopurinol. However, intraperitoneal administration at the same scheme did not produce any observable hypouricemic effect. These hypouricemic effects are partly due to the inhibition of XDH/XO activities in mouse liver. The pharmacological profile of the flavonoids is partly different from that of allopurinol. Such hypouricemic action and inhibition of the enzyme activity of quercetin and rutin may be responsible for a part of the beneficial effects of Biota orientalis extract on hyperuricemia and gout. The effects of quercetin and rutin on serum urate levels in hyperuricemic mice induced by oxonate and the inhibition of enzyme activities in mouse liver are discussed in relation to their absorption and metabolism, and their potential application to treat gout and hyperuricemia.