The peroxynitrite scavenging activity of a series of structurally related flavonoids was tested. It was found that flavonoids are excellent scavengers of peroxynitrite. Compared to the known peroxynitrite scavenger ebselen, the most active flavonoids proved to be 10 times more effective. Indications were found that the catechol group (ring B) and the hydroxyl group at position 3 give the highest contribution to the peroxynitrite scavenging effect. The peroxynitrite scavenging is discussed in relation to the beneficial effect of flavonoid intake on the incidence of coronary heart disease.