Initial ferricytochrome c (Cyt(III)c) reduction rates occurring in aerobic or anaerobic solutions containing either 3-nitrobenzothiazolo[3,2-a]-(NBQCl), 1-ethyl-3-nitrobenzimidazolo[3,2-a]-(ENBIQCl), 7-ethylbenzimidazolo[3,2-a]quinolinium chloride (EHBIQCL), or nitrofurantoin (NFT) and xanthine/xanthine oxidase were measured. Maximum rates in nitrogen-saturated solutions follow the order NFT > NBQCL > ENBIQCL > EHBIQCL. These rates correlate linearly with the half-wave reduction potentials (E1/2) of these compounds. With the exception of EHBIQCl, smaller rates of Cyt(III)c reduction were obtained in air-saturated than in nitrogen-saturated solutions at the quinolinium salt concentrations used. Larger concentrations of superoxide dismutase (SOD) are needed for 50% inhibition of the Cyt(III)c reduction reaction for heterocyclic compounds with larger E1/2 values. Thus, measurement of the portion of the Cyt(III)c reduction rate under air that is inhibited by SOD does not account solely for the production of superoxide. These observations suggest that NBQCL, ENBIQCl, and less probably EHBIQCl may interfere with mitochondrial energy metabolism or induce DNA damage through reduced intermediates.