Polycyclic aromatic hydrocarbons (PAHs) are a class of compounds considered to have human carcinogenic potential and have been found associated with many respirable, environmental particle pollutants. The effect of these ultrafine, insoluble, carrier particles on the lung retention and metabolic fate of inhaled PAHs was investigated with a radiolabeled model PAH, [3H]benzo(a)pyrene (3H-BaP). Fischer-344 rats were exposed (30 min) by nose-only inhalation to 3H-BaP adsorbed (approximately 0.1% by mass) onto diesel engine exhaust particles. These aerosols were generated in a dynamic aerosol generation system by vapor condensation methods. The total mass concentration of these aerosols was 4-6 micrograms/liter of air with a mass median diameter of 0.14 micron. Lung clearance of the inhaled particle-associated 3H radioactivity occurred in two phases. The initially rapid clearance of this inhaled radiolabel had a half-time of less than 1 hr. The second, long-term component of lung clearance had a half-time of 18 +/- 2 days and represented 50 +/- 2% of the 3H radioactivity that had initially deposited in lungs. In contrast, previous inhalation studies with a pure 3H-BaP aerosol showed that greater than 99% of the 3H radioactivity deposited in lungs was cleared within 2 hr after exposure (Sun et al., Toxicol. Appl. Pharmacol. 65, 231-244, 1982). By HPLC analysis, the majority of diesel soot-associated 3H radioactivity retained in lungs was BaP (65-76%) with smaller amounts of BaP-phenol (13-17%) and BaP-quinone (5-18%) metabolites also being detected. No other metabolites of BaP were detected in lungs of exposed rats. Tissue distribution and excretion patterns of 3H radioactivity were qualitatively similar to previous inhalation studies with 3H-BaP coated Ga2O3 aerosols (Sun et al., 1982). These findings suggest that inhaled PAHs may be retained in lungs for a greater period of time when these compounds are associated with diesel engine exhaust particles. In addition, these compounds retained in lungs can be metabolized in lungs. These results may have significant implications for the health risks that may be involved with human exposure to particle-associated organic pollutants.