The generation of 8-hydroxydeoxyguanosine (8OHdG) in calf thymus DNA treated with O-phenylphenol (OPP) or its major metabolites, phenylhydroquinone (PHQ) and phenylbenzoquinone (PBQ), was studied. The content of 8OHdG residues was increased in DNA treated with PHQ, and the generation of 8OHdG was highly dependent on PHQ concentration. PBQ had little effect on the formation of 8OHdG, and OPP had no effect. The formation of 8OHdG by PHQ was reduced by oxygen radical scavengers such as catalase, sodium benzoate and sodium azide. The PHQ-induced 8OHdG formation was accelerated by the addition of CuCl or CuCl2 to the reaction mixture, but was decreased by the addition of chelating agents such as EDTA, bathocuproinedisulfonic acid disodium salt (bathocuproine disulfonate) and O-phenanthroline. These results demonstrate that hydroxyl radicals generated in the process of oxidation of PHQ contribute to the formation of 8OHdG in DNA, and copper ions facilitate the oxidative DNA damage. Copper ions greatly accelerated the PHQ-induced DNA cleavage in vitro, although they had no effect on cleavage without PHQ. On the other hand, DNA cleavage occurred by the addition of FeCl2 in the absence and presence of PHQ. FeCl2 stimulates 8OHdG formation only slightly with or without PHQ. Furthermore, the stimulatory effect of FeCl2 on 8OHdG formation was observed even in the presence of EDTA. The formation of 8OHdG in bladder DNA is likely to be one of a series of events leading to bladder tumors seen in rats fed OPP-containing diet.