Evening primrose oil (Oenothera biennis) is a rich source of omega-6 series fatty acids. We report here the effects of dietary supplementation with evening primrose oil (EPO) on platelet aggregation as the main factor in arterial thrombus formation in an experimental model of atherogenesis in rabbits. A total of 40 male white New Zealand rabbits were divided into four groups (n = 10 animals/group): 1: normal diet, 2: atherogenic diet (ATD), 3: normal diet enriched with 15% EPO, 4: ATD + EPO. Each group was kept on the diet for 6 weeks. We determined serum lipid profile, platelet aggregation in whole blood, platelet thromboxane B2 production and platelet lipid peroxides. The atherogenic diet increased platelet aggregation (135% when ADP was used, and 185% when collagen was used as the inducer). Evening primrose oil reduced hyperaggregation to the values obtained in rabbits fed with the normal diet. Thromboxane synthesis was increased from 0.18 to 2.28 nmol/10(9) platelets); EPO reduced this value to 1.38 nmol/10(9) platelets. Lipid peroxides were increased by ATD from 0.27 to 0.81 nmol/10(8) platelets; EPO prevented this increase (0.35 nmol/10(8) platelets). In conclusion, EPO reduced platelet hyperaggregability in rabbits fed an atherogenic diet.