NAD(P)H-quinone oxidoreductase (EC 1. 6. 99. 2) was purified form S. cerevisiae. The enzyme readily reduced 2,6-dichlorophenolindophenol, a quinonoid redox dye, as well as substituted benzo- and naphthoquinones, and could accept electrons from either NADH or NADPH. The purified NAD(P)H-quinone oxidoreductase turned out to be capable of reducing nitrosoarenes as well as a variety of quinones. A chemical-trapping technique using 4-chloro-1-naphthol was used to show that the N,N-dimethyl-p-benzoquinonediiminium cation was produced in the reduction of 4-nitroso-N,N-dimethylaniline catalyzed by NAD(P)H-quinone oxidoreductase.