The effects of plant extracts on the activity of anti-oxidative enzymes in rats were investigated. Sprague-Dawley rats were fed with the extracts and fractions from Rhus javanica, Malus sieboldii, and Ostrya japonica, and the activities of anti-oxidative enzymes, such as superoxide dismutase (SOD) and catalase (CAT) were investigated. SOD activities in blood serum and liver increased following an injection of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (AAPH), and these increases were not cancelled out by feeding plant extracts. CAT activities in blood serum and liver increased following the AAPH injection, but these increases were reversed by feeding the extracts of R. javanica and O. japonica. This finding was similar to the result that the CAT activity increase induced by feeding alcohol was cancelled out by feeding ascorbic acid. These results suggest that anti-oxidative polyphenolics from these plants may act through the same mechanism as that of the well-known antioxidant ascorbic acid towards hydrogen peroxide.