The ability of the lung to mechanically remove inhaled deposited particles is an important mammalian defense mechanism that can be evaluated in the laboratory. Experiments that measure clearance kinetics have been performed by various investigators using human as well as large and small laboratory animals as subjects. Several agents have been shown to significantly alter clearance phenomena in the lung. This paper describes quantitative clearance experiments that used radioactive tracer particles to assess lung damages after exposure of rats to ozone, a photochemical air-pollutant gas. Radioactively labeled tracer microspheres were inhaled by groups of 30 rats prior to exposure to ozone. Ozone levels studied were 0.4, 0.8, and 1.0 ppm and all exposures were 4 hr in length. These exposures caused a delay in the early (0-50 hr postdeposition) clearance and an acceleration in the late (50-300 hr postdeposition) clearance rate of the tracer particles. Dose response curves show that clearance was affected more by the higher concentrations of ozone.