The aims of the present study were to determine the level of (-)-epicatechin (EC) and its metabolites in rat plasma after oral administration of cocoa powder and to evaluate the protective effect of cocoa powder in terms of suppressing the oxidation of plasma components. Rats were orally administered 1 g cocoa powder/kg body weight, containing 7·80 mg EC, and their blood was collected before administration and at designated time intervals thereafter. The EC and its metabolites in plasma were treated with β-glucuronidase and/or sulfatase, then analysed by HPLC and by liquid chromatography–MS. Several EC-related compounds were detected in plasma such as free EC, and glucuronide, sulfate, and glucuronide–sulfate conjugates of non-methylated or methylated EC. All EC metabolites showed a maximum concentration in plasma at 30–60 min post-administration. Glucuronide conjugates of both non-methylated and methylated EC were found in high concentration in plasma. Moreover, administration of cocoa powder significantly reduced the accumulation of lipid peroxides in plasma and significantly reduced the consumption of α-tocopherol in plasma oxidized by treatment with 2,2′-azobis-(2-amidinopropane) dihydrochloride (AAPH (25 mmol/l)) or CuSO4 (100 μmol/l) compared with that in the case of plasma obtained before administration. The total EC concentration in plasma was negatively correlated with the level of accumulation of lipid peroxides in plasma oxidized by treatment with AAPH (25 mmol/l) and was positively correlated with the level of residual α-tocopherol in plasma oxidized by treatment with CuSO4 (100 μmol/l). These results indicate that EC in cocoa powder was absorbed from the digestive tract, that various conjugated forms of EC were generated in the digestive tract and distributed to the plasma, and that these enhanced the antioxidative activity of plasma.