Hydralazine caused site-specific DNA damage in the presence of Cu(II), Co(II), Fe(III), or peroxidase/H2O2. The order of inducing effect of metal ions on hydralazine-dependent DNA damage [Cu(II) greater than Co(II) greater than Fe(III)] was related to that of accelerating effect on the O2 consumption rate of hydralazine autoxidation. Catalase completely inhibited DNA damage by hydralazine plus Cu(II), but hydroxyl radical (.OH) scavengers and superoxide dismutase did not. On the other hand, DNA damage by hydralazine plus Fe(III) was inhibited by catalase and .OH scavengers. Hydralazine plus Cu(II) induced piperidine-labile sites predominantly at guanine and some adenine residues, whereas hydralazine plus Fe(III) caused cleavages at every nucleotide. Activation of hydralazine by peroxidase/H2O2 caused guanine-specific modification in DNA. ESR-spin trapping experiment showed that .OH and superoxide are generated during the Fe(III)- or Cu(II)-catalysed autoxidation of hydralazine, respectively, and that nitrogen-centered radical is generated during the Cu(II)- or peroxidase-catalysed oxidation. The generation of nitrogen-centered radical was also supported by HPLC-mass spectrometry. The results suggest that the guanine-specific modification by the enzymatic activation of hydralazine is due to the nitrogen-centered hydralazyl radical or derived active species, whereas .OH participates in DNA damage by hydralazine plus Fe(III). The mechanism of hydralazine plus Cu(II)-induced DNA damage is complex. The possible role of the DNA damage induced by hydralazine in the presence of Cu(II) or peroxidase/H2O2 is discussed in relation to hydralazine-induced lupus, mutation, and cancer.