DNA adduct formation of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) has been investigated by 32P-postlabeling. Similar adduct profiles were observed from calf thymus DNA modified in vitro with the putative carcinogenic metabolite N2-acetoxyamino-3-methylimidazo[4,5-f]-quinoline (N-acetoxy-IQ) and from hepatic DNA of rats treated with IQ. N-(Deoxyguanosin-8-yl)-2-amino-3-methylimidazo[4,5-f]quinoline (dG-C8-IQ) accounted for approximately 90% of the total adducts observed in calf thymus DNA under postlabeling conditions where ATP was limiting; however, 5-(deoxyguanosin-N2-yl)-2-amino-3-methylimidazo[4,5-f]quinoline (dG-N2-IQ) was detected only when DNA was labeled with excess ATP. Under these labeling conditions, dG-C8-IQ and dG-N2-IQ accounted for approximately 75% and 7% of the total adducts, respectively. Five other spots accounted for the remaining radioactivity. Comparable results were obtained from rat liver DNA. Following DNA adduct enrichment by solid phase extraction, dG-C8-IQ and dG-N2-IQ accounted for 60–76% and 10–13%, respectively, of the total adducts in rat liver. The adduct profiles obtained from reaction of 2′-deoxyguanosine 3′-monophosphate (dG-3′-PO4-) with the photoactivated azide derivative of IQ, 2-azido-3-methylimidazo[4,5-f]-quinoline (N3-IQ), were qualitatively similar to those obtained by reaction with N-acetoxy-IQ. The C-8 and N2 adducts were the only reaction products detected. The reactivity and sites of adduct substitution were dependent upon solvent conditions and pH, with increasing adduct formation under alkaline pH. The chemical reactivity of photoactivated N3-IQ with dG-3′-PO4− was significantly greater than that of N-acetoxy-IQ when reactions were conducted in water, in citrate buffer (pH 5.0), or in phosphate buffer (pH 7.4). Increased reactivity was attributed to increased levels of dG-C8-IQ adduct formation, except for reactions conducted in citrate buffer (pH 5.0), where there was a proportional increase in both C-8 and N2 guanine adducts. However, the chemical reactivity of these two IQ derivatives and their sites of dG substitution were identical when the reactions were conducted in phosphate buffer (pH 9.0). The ratio of the dG-N2-IQ adduct to the total adducts increased at alkaline pH in reactions involving N3-IQ, but the ratio was not affected by a change in the pH of the medium for reactions with N-acetoxy-IQ. The ratio of the dG-N2-IQ adduct to the total adducts also increased as a function of phosphate concentration for reactions involving both N-acetoxy-IQ and N3-IQ. Formation of the ring substituted dG-N2-IQ adduct indicates that nitrenium and carbenium ion formation occurred for both N-acetoxy-IQ and photoactivated N3-IQ. The reactivity and the effects of pH and solvent on sites of adduct substitution are distinct for these two chemically reactive derivatives of IQ and suggest that the mechanism of adduct formation is not identical.