1‐Nitropyrene (NP), as found in the environment, is more typically associated with carbonaceous particles than found as an aerosol of the pure compound. To determine whether (and why) an association with particles resulted in prolonged lung retention of NP, rats were exposed to14C‐NP as a pure aerosol or adsorbed on carbon black particles. Total14C retained in the lung was greater at all times from 0.5 h to 30 d after exposure to14C‐NP adsorbed to carbon black particles than after exposure to pure14C‐NP (p< .05). The fraction of total14C in lung bound to carbon black particles decreased steadily with time after exposure, indicating in vivo removal of NP from the particles. At 0.5 h after exposure, the fraction of the estimated deposited14C that was covalently bound to lung macromolecules was twofold greater for NP adsorbed on carbon black than for pure NP. Covalently bound14C in lungs increased with time after exposure to14C‐NP adsorbed to carbon black, reaching levels of approximately 1% of the deposited radioactivity at 7–30 d after exposure, whereas levels of covalently bound14C declined with time after exposure to pure NP. Thus, at 30 d after exposure, the amount of14C covalently bound to lung macromolecules was approximately 10‐fold greater (p< .05) in rats that inhaled14C‐NP adsorbed on carbon black particles than in rats that inhaled pure14C‐NP aerosols. These results suggest that association of NP with carbon black particles augments the interaction of reactive metabolites of NP with target macromolecules. This phenomenon is thought to be related to the slow release of NP from carbon black particles, and may augment the biological effects of inhaled NP when adsorbed on carbon black or similar particles in the environment.