The effect of continuous exposure to diluted diesel exhaust on the pulmonary retention of inhaled diesel particles was studied in male Fischer 344 rats. Test animals were first exposed to clean air or diluted diesel exhaust in exposure chambers at nominal particulate concentrations of 250 micrograms/m3 or 6 mg/m3 for 20 hr/day, 7 days/week, for periods lasting from 7 to 112 days, followed by a nose-only exposure to 14C-tagged diesel particles for 45 min. At preselected time intervals after the radioactive exposure, the 14C-activities in the lungs of groups of four animals were measured to determine the clearance of the 14C-diesel particles up to 1 year. The pulmonary retention of the radioactive diesel particles was greater in animals which had been preexposed to diesel exhaust. The slower alveolar clearance of particle-laden macrophages and leukocytes can be described by a normal biphasic clearance model. Since some of the macrophages were found sequestered as aggregates in the pulmonary region, a slow-clearing residual component was included in a modified lung retention model. When these residual fractions were determined and excluded from the active particulate transport within the lungs, normal alveolar clearance rates were calculated for the animals with a preexposure diesel particulate dose less than 0.8 mg. Slower clearance was observed at a dose of 6.5 mg and no clearance was evident at a dose of 11.8 mg in their lungs.(ABSTRACT TRUNCATED AT 250 WORDS)