Due to their ubiquitous distribution, flavonoids from different classes are commonly present together in foods. However, little is known about the interactions between them. The flavonol quercetin and the flavan-3-ol (+)-catechin are among the most abundant flavonoids in the diet. In the present study, we have analysed the interactions between these two flavonoids on vascular function using two pure compounds and mixtures of these flavonoids in 1:0·1, 1:1 or 1:10 proportions. Quercetin induced a more potent concentration-dependent relaxant effect than catechin in the isolated rat aorta, and the isobolographic analysis of the mixtures showed no synergistic or antagonistic effects between them, i.e. their effects were additive. Quercetin was more potent in mesenteric than in pulmonary arteries. Catechin had weak effects in these vessels and did not modify the effects of quercetin. Endothelial dysfunction induced by increased oxidative stress by the superoxide dismutase inhibitor diethyldithiocarbamate was prevented by quercetin, whereas catechin showed a weak effect and the 1:1 mixture an intermediate effect compared with the pure compounds. Quercetin but not catechin showed a pro-oxidant and NO-scavenging effect, which was not prevented by catechin. In conclusion, catechin was less potent than quercetin as a vasodilator, pro-oxidant or to prevent endothelial dysfunction, and there were no synergistic interactions between quercetin and catechin.