Besides a variety of quinones, purified bovine liver quinone reductase catalyzed the reduction of N,N-p-nitrosoaniline to N,N-dimethyl-p-phenylenediamine. The formation of N,N-dimethyl-p-phenylenediamine was identified by TLC, GC, GC-MS and NMR. Quinone reductase can utilize either NADH or NADPH as a source of reducing equivalents. The apparent Km for 1,4-benzoquinone and N,N-dimethyl-p-nitrosoaniline was 1.64 mM and 0.22 mM, respectively The reduction of N,N-dimethyl-p-nitrosoaniline was almost entirely hampered by dicumarol or Cibacron blue 3GA, potent inhibitors of mammalian quinone reductase. During the bovine liver quinone reductase-catalyzed reduction of N,N-dimethyl-p-nitrosoaniline, benzoquinonediiminium ion was produced.