The fate of inhaled diesel exhaust particles was studied in male Fischer 344 rats and Hartley guinea pigs using radioactive diesel particles, tagged in the insoluble particulate core with 14C and generated from a single-cylinder diesel engine. The potential artifact of increased radioactivity due to the absorption of 14CO2 in the blood was minimized by passing the exhaust through a diffusion scrubber prior to its dilution and introduction into a nose-only exposure chamber. Disappearance of the inhaled 14CO2 from blood through the expired air and urine was rapid, indicating that a correction for the increased radioactivity was necessary only for tissue samples generated during the first day after the exposure. An atomic absorption spectrophotometric method was developed to determine the amount of blood and, thus, its contribution of 14CO2 activity in excised organs and tissues. Fischer rats exposed to diluted diesel exhaust at 2 particulate concentrations with similar total inhaled dose (7 mg/m3 for 45 min, and 2 mg/m3 for 140 min) had comparable deposition efficiencies and showed no significant difference in particle clearance for data measured up to 1 yr after the exposure. Long-term retention of inhaled diesel particles in Fischer rats, measured up to 330 d after the exposure, was analyzed as 3 clearance phases with half-times of 1 d, 6 d, and 80 d, respectively. In contrast, very little clearance was observed in guinea pigs between d 10 and d 432 after the exposure, and only the early clearance phase can be represented by a single exponential curve with a half-time of 1-2 d.