Moutan Cortex (root cortex of Paeonia suffruticosa ANDREWS) and Paeoniae Radix (root of Paeonia lactiflora PALLAS) are crude drugs used in many traditional prescriptions and have constituents in common. We studied the effects of extracts of these crude drugs and their constituents on oxidative DNA damage caused by phenylhydroquinone (PHQ), a major metabolite of o-phenylphenol. Both drugs suppressed the cleavage of pUC18 DNA induced by PHQ, and scavenged the superoxide and hydroxy radical generated by the chemical. They also inhibited the oxidative DNA cleavage by tert-butylhydroquinone (TBHQ), one of the major metabolites of butylated hydroxyanisole. When constituents were examined with the same system, galloylpaeoniflorin and 1,2,3,4,6-penta-O-galloyl-beta-D-glucose (PGG) were found to be the most potent inhibitors of the DNA cleavage. These constituents had oxygen radical scavenging activity. Paeonol also attenuated the DNA cleavage. Paeoniflorin and albiflorin had relatively small inhibitory effects on DNA cleavage. However, catechin enhanced the PHQ-induced DNA cleavage. The suppression of oxidative DNA damage by Moutan Cortex and Paeoniae Radix might be attributable to the additive effects of galloylpaeoniflorin, PGG and other constituents.