Flavonoids might exert modulatory effects in cells independent of classical antioxidant capacity throught selective actions at different levels. This study was performed to investigate the effects of two flavonols (Kaempferol and Quercetin) and two flavan-3ols (Catechin and Epicatechin) on the XO and Mn-SOD activity, in order to better define their role in a variety of oxidant mediated diseases. In addition, the influence on the enzyme activities of the carboxyl group at the C4 position of heterocycle conjugated with C2-C3 double bound was evaluated by a spectrophotometric assay using the xanthine/XO/XTT system. Quercetin, 100mM, shows a pro-oxidant effect lower than Kaempferol. At lowest concentration (1-10mM), both these compounds appear to exert a antioxidant action by decreasing the superoxide radical level. This effect in a range of concentration between 10-1 mM is dose-depending and inversely correlated to the doses challenged. A further decrease in superoxide radicals concentration occurs after the addition of SOD in the reaction system. Conversely, Catechin and Epicatechin reduce the superoxide radicals production at all tested concentrations thus eliciting a strong scavenging activity. Our study shows that marked differences can be observed between flavonoids with different structural characteristics in their ability to inhibit or to stimulate superoxide radicals production. These data further demonstrate the opposite dose-dependent effects of flavonols on XO and Mn-SOD. These results provides new informations for study of these compounds as therapeutic agents in oxidative stress-related diseases.