The plasma membrane oxidoreductase (PMOR) activity, which mainly utilises ascorbate as intracellular electron donor, represents a major mechanism for cell-dependent reduction of extracellular oxidants and might be an important process used by the erythrocytes to keep a reduced plasma environment. We previously reported that in human erythrocytes, myricetin and quercetin act as intracellular substrates of a PMOR showing a novel mechanism whereby these flavonoids could exert beneficial effects under oxidative stress conditions. Here, we evaluated the ability of different flavonoids (quercetin, myricetin, morin, kaempferol, fisetin, catechin, luteolin, apigenin, acacetin, rutin, taxifolin, naringenin, genistein) and of two in vivoO-methylated metabolites of quercetin (isorhamnetin and tamarixetin) to be substrates of PMOR, by comparing their antioxidant capacity (i.e. direct interaction with the oxidant ferricyanide or with the free radical 1,1-diphenyl-2-picryl-hydrazil) with their ability to penetrate the erythrocytes and donate electrons to the PMOR. The results obtained indicate that, although most of the flavonoids display significant antioxidant activities, only those (quercetin, myricetin, fisetin) that combine the cathecol structure of the B ring (responsible for the reducing activity) with the 2,3 double bond and 4-oxo function of the C ring (responsible for the uptake by erythrocytes) can act as intracellular substrates for PMOR. It is of note that the metabolites of quercetin enter erythrocytes and donate electrons to the PMOR as the parent compound. The present data show a relationship between the flavonoid structures and their ability to provide electrons to the PMOR, suggesting an additional mechanism whereby dietary flavonoids may exert beneficial effects in man.