This study was designed to determine whether incorporation of gamma-tocotrienol or alpha-tocopherol in an atherogenic diet would reduce the concentration of plasma cholesterol, triglycerides and fatty acid peroxides, and attenuate platelet aggregability in rats. For six weeks, male Wistar rats (n = 90) were fed AIN76A semisynthetic test diets containing cholesterol (2% by weight), providing fat as partially hydrogenated soybean oil (20% by weight), menhaden oil (20%) or corn oil (2%). Feeding the ration with menhaden oil resulted in the highest concentrations of plasma cholesterol, low and very low density lipoprotein cholesterol, triglycerides, thiobarbituric acid reactive substances and fatty acid hydroperoxides. Consumption of the ration containing gamma-tocotrienol (50 mg/kg) and alpha-tocopherol (500 mg/kg) for six weeks led to decreased plasma lipid concentrations. Plasma cholesterol, low and very low density lipoprotein cholesterol, and triglycerides each decreased significantly (P < 0.001). Plasma thiobarbituric acid reactive substances decreased significantly (P < 0.01), as did the fatty acid hydroperoxides (P < 0.05), when the diet contained both chromanols. Supplementation with gamma-tocotrienol resulted in similar, though quantitatively smaller, decrements in these plasma values. Plasma alpha-tocopherol concentrations were lowest in rats fed menhaden oil without either chromanol. Though plasma alpha-tocopherol did not rise with gamma-tocotrienol supplementation at 50 mg/kg, gamma-tocotrienol at 100 mg/kg of ration spared plasma alpha-tocopherol, which rose from 0.60 +/- 0.2 to 1.34 +/- 0.4 mg/dL (P < 0.05). The highest concentration of alpha-tocopherol was measured in plasma of animals fed a ration supplemented with alpha-tocopherol at 500 mg/kg.