The aim of this work is to study the effect of the flavonoids rutin and quercetin on hepatic monooxygenase activities in experimental influenza virus infection (EIVI). EIVI causes oxidative stress in the whole organism. This is confirmed by the rapidly increased concentrations of thiobarbituric reactive substances in influenza-infected mice: lungs - 290%; blood plasma - more than 320%; liver - 230%; brain - 50%. Although known for their antioxidant activities, rutin and quercetin exhibit prooxidant effect in healthy and antioxidant activity in influenza-infected animals. The pretreatment with both flavonoids (20 mg/kg b.w.) restores oxidative damage mostly in the target organ of the infection as well as in the liver of all infected mice (lungs: rutin - 30%, quercetin - 40%, combination - 45%; liver: rutin - 12%; quercetin - 40%; combination - 50%). As far as EIVI causes oxidative stress, toxicosis and inhibition of the hepatic monooxygenase activity, it is important to study the effects of rutin and quercetin on these systems. Both flavonoids induce the level of cytochrome P-450 (rutin - 13%, quercetin - 30%, combination - 22%) but inactivate NADPH-cytochrome c reductase, aminopyrine N-demethylase and analgin N-demethylase on the 5th day of EIVI. Probably, these flavonoids affect different components of the monooxygenase system. These effects could be explained with oxidative hepatic intoxication on the 5th critical day of EIVI as well as higher dose treatment. More data are needed on the antioxidant/prooxidant effects of rutin and quercetin, probably due to specific metabolic and physiological activities, chemical structure, etc.