A B S T R A C T Formation of lipid peroxides rises sharply when platelets undergo the release reaction. In this study the in vitro effect of vitamin E on platelet aggregation was investigated. a-Tocopherol, an antioxidant of known inhibitory action on lipid peroxidation, was added to platelet suspensions in concentrations up to 1.5 mM. A dose-dependent reduction in platelet aggregation was observed, with complete inhibition of the secondary wave of aggregation at > 0.9 mM atocopherol. The inhibitory effect of a-tocopherol on the platelet release reaction was further documented by the decrease in aggregation-induced release of [14C]5-hydroxytryptamine from prelabeled platelets and by the reduction of N-acetylglucosaminidase activity released into the medium. The sharp rise in lipid peroxides normally associated with platelet aggregation was markedly reduced by a-tocopherol and also by acetylsalicylic acid, a known inhibitor of the platelet release reaction.In vivo studies examined the effect of oral vitamin E administration (1,200-2,400 IU daily) on plasma and platelet levels of a-tocopherol. Up to 1,800 IU daily, increasing dosages of vitamin E resulted in increasing concentrations of a-tocopherol in plasma and platelets, but intake of vitamin E in excess of this dosage failed to show any further increase in plasma or platelet levels.