The microbial metabolism of the mutagenic and carcinogenic polycyclic nitroaromatic hydrocarbon, 1-nitropyrene, has been studied. Under anaerobic conditions, bacterial suspensions from rat intestinal contents converted 1-nitropyrene to one major and two minor metabolites. The rate of metabolism by rat intestinal microflora (10(9) bacteria/ml) was rapid with greater than 90% conversion occurring within 1 h. The major metabolite was identified as 1-aminopyrene through high pressure liquid chromatographic and mass spectral comparisons with an authentic standard. The suspected metabolites, N-acetyl-1-aminopyrene, 1-nitrosopyrene, N-hydroxy-1-aminopyrene, and 3-, 6-, and 8-hydroxy-1-nitropyrene were synthesized, but these derivatives were not detected in culture extracts from rat intestinal contents incubated anaerobically with 1-nitropyrene. Nine genera of anaerobic and facultative bacteria normally associated with the intestine also converted 1-nitropyrene to 1-aminopyrene. These data indicate that a wide range of intestinal bacteria are able to reduce 1-nitropyrene. The results are discussed in relation to the in vivo metabolism of this polycyclic nitroaromatic hydrocarbon.