Flavonols (quercetin and rutin) and flavanes (cyanidol and meciadonol) were studied for their effect on non-enzymatic lipid peroxidation, lipoxygenase and cyclo-oxygenase activities, binding to albumin and platelet membranes. These biochemical properties of four flavonoids were compared with respect to their antithrombotic action in vivo and their efficacy at influencing the platelet-endothelium interaction in vitro. All four flavonoids inhibited the ascorbate-stimulated formation of malondialdehyde by boiled rat liver microsomes (quercetin greater than rutin approximately cyanidol approximately meciadonol) and inhibited platelet lipoxygenase activity (quercetin greater than cyanidol greater than meciadonol greater than rutin) whereas only flavonols, but not flavanes, stimulated cyclo-oxygenase and were bound to platelet membranes. The same two flavonols dispersed platelet thrombi which were adhering to the rabbit aortic endothelium in vitro (EC50 for quercetin was 80 nM and for rutin 500 nM) and prevented platelets from aggregation over blood-superfused collagen strip in vivo (ED50 for quercetin was 5 nmol/kg and for rutin 33 nmol/kg i.v.). Cyanidol and meciadonol were not effective as anti-thrombotic agents. It is concluded that activated platelets adhering to vascular endothelium generate lipid peroxides and oxygen-free radicals which inhibit endothelial biosynthesis of prostacyclin and destroy endothelium-derived relaxing factor (EDRF). Flavonols are anti-thrombotic because they are selectively bound to mural platelet thrombi and owing to their free radical scavenging properties resuscitate biosynthesis and action of endothelial prostacyclin and EDRF. Thus, flavonols release the thrombolytic and vasoprotective endothelial mediators only in these vascular segments which are covered by a carpet of aggregating platelets.