The effects of exposures to sulfur dioxide and formaldehyde atmospheres on the clearance of inhaled, insoluble tracer particles from the lungs of rats have been studied. The tracer particles employed were polystyrene latex microspheres radio‐labeled with51Cr. Following the deposition of the 1.9‐μm activity median aerodynamic diameter (AMAD) particles, the rats were divided into 3 groups for a single 4‐h exposure to purified air, 20 ppm sulfur dioxide, or 20 ppm formaldehyde. Early, presumably upper‐respiratory‐tract, clearance was monitored by analysis of radioactivity excreted in feces, while late, presumably deep‐lung, clearance was followed by thoracic counting of the animals. Both the sulfur dioxide and formaldehyde atmospheres did significantly delay early clearance (p < 0.1, two‐tailed t‐test). However, the late clearance rates of the two pollutant‐exposed groups of rats were not significantly different from that of the purified air‐exposed group of rats. Although sulfur dioxide had numerically greater effects than formaldehyde, the differences were not statistically significant at the p < 0. 7 level.