In vivo 4-hydroxyamino[2-3H]quinoline 1-oxide-modified DNA and in vitro 4-acetoxyamino[2-3H]quinoline 1-oxide-modified DNA were enzymatically hydrolyzed, and the hydrolysates were analyzed by high-performance liquid chromatography. The two patterns were compared, and we showed that all of the high-performance liquid chromatography peaks which were recovered from in vivo-modified DNA were present in the hydrolysate of in vitro-modified DNA. Therefore, we used the in vitro 4-acetoxyamino[2-3H]quinoline 1-oxide-modified DNA to investigate the quinoline-purine adducts which are characteristics of the mode of action of the carcinogen 4-nitroquinoline 1-oxide. By comparison with the enzymatic hydrolysates of 4-acetoxyamino[2-3H]quinoline 1-oxide-modified covalent poly(deoxyadenylate-deoxythymidylate) X poly(deoxyadenylate-deoxythymidylate) and covalent poly(deoxyguanylate-deoxycytidylate) X poly(deoxyguanylate-deoxycytidylate) three nitroquinoline adducts were enumerated on the modified DNA. One of them was previously characterized as a C8-guanyl adduct. We proved that the two other are a guanine and an adenine adduct, respectively. A quinoline derivative was identified in the hydrolysates of the in vivo- and in vitro-modified DNAs as 4-aminoquinoline 1-oxide, the origin of which was postulated to be a degradation compound of one (or more) adduct(s). Moreover, the presence of two degradation compounds of the C8-guanyl adduct was shown in mild alkaline conditions. We suspected an imidazole ring-opened form.