The ability of the isomeric quinone metabolites of benzo[a]pyrene, benzo[a]pyrene-6,12-dione, benzo[a]pyrene-1,6-dione, and benzo[a]pyrene-3,6-dione to undergo reversible, univalent oxidation-reduction cycles involving the corresponding benzo[a]pyrenediols and intermediate semiquinone radicals has been characterized. Under anaerobic conditions, all three benzo[a]pyrenediones are easily reduced to benzo[a]pyrenediols, even by mild biological agents such as NAD(P)H, cysteamine, and glutathione. The benzo[a]pyrenediols, in turn, are very rapidly autoxidized to the benzo[a]pyrenediones when exposed to air. Substantial amounts of hydrogen peroxide are produced during these autoxidations, and other reactive reduced oxygen species, such as the superoxide and hydroxyl radicals, are probably formed transiently as well. The benzo[a]pyrenediol-benzo[a]pyrenedione interconversions proceed by one-electron steps; the corresponsing semiquinone radicals can be monitored by electron spin resonance spectroscopy as inter mediates during these reactions carried out at high pH. Benzo[a]pyrenediones induce DNA strand scission when incubated with bacteriophage T7 DNA. This damage is modified by conditions which indicate that reduced oxygen species propagate the free-radical reactions responsible for the strand scission. Benzo[a]pyrenediones are electron-acceptor substrates for NADH dehydrogenase from Clostridium kluyveri. Catalytic amounds of these benzo[a]pyrene metabolites, together with this respiratory enzyme function as cyclic oxidation-reduction couples which link NADH and molecular oxygen in the continuous production of hydrogen peroxide. These data, together with preliminary results with cells in culture, indicate that benzo[a]pyrenediones are potentially harmful metabolites of benzo[a]pyrene, acting by processes which lead to their regeneration rather than depletion; nucleic acid and call damage is probably produced by the reactive reduced oxygen species resulting from such regenerative oxidation-reduction cycles.