AZQ had been found to produce DNA strand breaks and interstrand cross-links in intact cells; evidence had indicated that these two DNA lesions arise by different chemical mechanisms and vary independently in degree in different cell types. In the present work, the mechanisms of the production of DNA strand breaks and interstrand cross-links by AZQ were studied in isolated cell nuclei. This system avoided the problem of poor penetration of test substances into cells. The DNA lesions were measured by means of the alkaline elution technique. It was found that the production of DNA strand breaks by AZQ in isolated nuclei required the addition of a reducing agent such as NADPH and was almost completely prevented by superoxide dismutase. This indicates that the mechanism of DNA strand breakage involves transfer of an electron from a reduced form of AZQ to molecular oxygen. Unexpectedly, interstrand cross-linking also was enhanced greatly by previous reduction of AZQ by NADPH or NaBH4. However, this reaction was not inhibited by superoxide dismutase. General alkylating activity of AZQ also was stimulated by reduction; the pH-dependence of this reaction was determined. The mechanism of DNA interstrand cross-linking by AZQ was surmised to stem from alkylation reactions of the two aziridine groups. The findings suggest the possibility that AZQ or related compounds may function as bioreductive alkylating agents which might be selectively toxic to hypoxic tissues.