Nitroarenes are widely distributed not only in the external atmosphere chiefly as the result of emissions from diesel-powered cars and power stations (i.e., fly ash) but also in the home and office environments because of their presence in the side streams of cigarette smoke. There is currently great interest in elucidating the basis of the biological properties of the nitroarenes, as they are powerful microbial mutagens, they induce gene mutations, genotoxic effects and neoplastic transformations in mammalian cells and they cause cancer in rodents (Rosenkranz and Mermelstein, 1983). The mutagenicity of nitroarenes in bacterial cells has been related to their ability of being reduced to arylhydroxylamines (Rosenkranz and Mermelstein, 1983) and possibly of being esterified to hydroxamic acid esters (E1-Bayoumy and Hecht, 1982; Fu et al., 1982; McCoy et al., 1982, 1983a, c). In mammalian cells, in addition to nitroreduction, ring oxidation may be the preferred mode of bioactivation (Rosenkranz et al., 1983). This oxidation may or may not be followed by nitroreduction. The diverse genetic effects induced by nitroarenes in mammalian cells have been attributed to differences in their metabolic capabilities, primarily a reflection of the fact that upon continued passage some cells may exhibit greatly decreased levels of the relevant monooxygenases. In spite of the near uniformity of positive mutagenic and genotoxic responses of the nitroarenes in mammalian cells, the results with yeast have been ambiguous. Using two different experimental protocols, McCoy and associates (1983b) were unable to demonstrate the genotoxicity of nitropyrenes for Saccharomyces cerevisiae D4. On the other hand, Wilcox and Parry (1981) and Wilcox et al. (1982) reported the recombinogenicity of 1,6and 1,8-dinitropyrene for Saccharomyces cerevisiae JD1. These investigators found that maximal activity was observed when the oxygen tension was decreased. This has led to the suggestion (Rosenkranz and Mermelstein, 1983) that reduced aerobiosis could result in the activation of oxygen-sensitive nitroreductases. Indeed, both in mammalian and bacterial cells this has been shown to occur (Biaglow, 1981; McCaUa, 1981; Peterson et al., 1979). The present investigation was undertaken to study further the assumption that the inactivity of nitropyrenes in Saccharomyces cerevisiae D4 under aerobic conditions was due to a deficiency in a functional nitroarene reductase. Two experimental approaches were taken: (1) nitropyrene reductase activity was determined under aerobic and anaerobic conditions and (2) advantage was taken of the availability of 1-nitrosopyrene (Howard et al., 1983a). It has been shown that like 1-nitropyrene (Howard et al., 1983b), 1-nitrosopyrene also forms adducts at the C-8 position of guanine, i.e., N-(deoxyguanosin-