The particulate exhaust of diesel engines consists of 0.1--0.2 micrometers mass median diameter particles composed of a carbonaceous core and adsorbed organic compounds. A technique was needed to determine accumulated lung burdens of particles in animals exposed to diesel exhaust as a determinant of dose. A method was developed for determining lung burdens of diesel soot particles in rats at 1 day, and at 1, 5, 15, 33 and 52 weeks after cessation of a subchronic exposure to diluted diesel exhaust. Lung tissue was dissolved in tetramethylammonium hydroxide and the diesel soot separated by centrifugation. The soot was suspended in water by sonication and the light absorption of samples was compared to standard suspensions of diesel soot. Recovery from lungs spiked with 50-1000 micrograms of soot was 89 +/- 5%. Rats exposed over a period of 18 weeks to diluted diesel exhaust at average net diesel particles concentrations of 150, 940 and 4100 micrograms/m3 had lung burdens of 35, 220 and 1890 micrograms/g lung, respectively, one day after the last exposure. The long-term clearance rates of soot had estimated half-times of 87 +/- 28, 99 +/- 4 days, for the low and medium exposure groups, respectively. The clearance half-time for the high level exposure group of 165 +/- 8 days was significantly longer (P less than 0.0001) than those of the other two groups.