Cocoa can contain a high concentration of flavanols and procyanidins which have been reported to have strong antioxidative activity. In the present study, male Sprague-Dawley rats were fed diets containing 0, 0.5, 1, or 2% cocoa rich in flavanols for two weeks. Blood, liver, heart and testes were collected and analyzed for markers of oxidative damage. Plasma epicatechin concentrations, 8-hydroxy-2'-deoxyguanosine (8OH2'dG), and oxidized and reduced glutathione were quantitated by HPLC with electrochemical detection. Plasma F(2)-isoprostanes were measured using an enzyme immunoassay. Plasma epicatechin concentrations increased in a dose-dependant fashion according to the amount of cocoa in the diet (128 nM-790 nM). Cocoa supplementation was associated with lower than normal concentrations of 8OH2'dG in the testes (0.590 + 0.40 vs. 0.328 + 0.29; p < 0.05). Liver and heart 8OH2'dG levels were unaffected by dietary treatment. In erythrocytes, the glutathione pool was significantly less oxidized in the cocoa fed group compared to controls (p < 0.05). In liver and testes, no differences in superoxide dismutase activities were detected. Concentrations of plasma F(2)-isoprostanes and thiobarbituric acid reactive substances were similar in all groups. These results support the concept that a diet rich in flavanols and procyanidins can improve oxidant defense and reduce tissue markers for oxidative stress, although these effects can be tissue specific.