The effects of chronic intake of dietary alcohol on myocardial peroxidation (measured as formation of diene conjugates), reduced glutathione content, and morphology and the protective actions of different antioxidant compounds (vitamin E and (+)-cyanidanol-3) were studied in rats. Alcohol, comprising more than 30% of the dietary calorie content, was administered to rats for six weeks. Compared with the controls, the left ventricle of the alcoholic animals had an increased diene conjugate content (5.4(0.5) vs 4.3(0.6) optical density X g wet weight-1) and a slightly, but not significantly, decreased glutathione content (1.62(0.05) vs 1.66(0.07) mumol X g wet weight-1). Simultaneous administration of antioxidants (vitamin E or (+)-cyanidanol-3) prevented the pathological changes in diene conjugates and significantly increased the glutathione content compared with the alcoholic rats. Electron microscopy showed remarkably few ultrastructural abnormalities in the myocardium of alcoholic animals fixed by vascular perfusion. The data are consistent with the hypothesis that reactive oxygen radicals are involved in the ethanol induced biochemical changes and that the antioxidants could prevent the increased formation of peroxides in the myocardium.