Nitropolycyclic aromatic hydrocarbons (nitroarenes), including 6‐nitrobenzo[a]pyrene (6‐NBap), occur in our environment and are mutagenic in bacterial mutagenesis assays. The mutagenicity of 6‐NBaP is enhanced when rat liver S9 is added. To investigate the cause of this increased activity, the metabolism of 6‐NBaP was carried out with a total rat liver homogenate obtained from 3‐methylcholanthrene‐ (MC‐) induced rats, a 9000 × g supernatant enzyme, and with both unwashed and washed microsomes. Ring‐hydroxy‐lated 6‐NBaP was detected. On the basis of retention times for known standards in a high‐performance liquid chromatographic system, benzo[a]pyrene (BaP) and 6‐acetoxy‐BaP (6‐OAcBaP) were isolated as products. BaP was further characterized via ultraviolet (UV) and mass spectra and 6‐OAcBaP by UV, mass, and nuclear magnetic resonance (NMR) spectra. 6‐HydroxyBaP (6‐OHBaP) was also detected by UV and mass spectra. It is suggested that BaP is formed via a nitroanion radical of 6‐NBaP and undergoes metabolism, while the 6‐OHBaP is acetylated to form 6‐OAcBaP. The acetyl donor remains to be identified.