This study examined age-related differences in the physiological responses of rats to inhaled automotive emissions. Previous reports suggested that lung development of animals exposed to oxidant gases early in life might be impaired, or that developing lungs might be more susceptible than adult lungs to inhaled toxicants. There were no previous comparisons in developing and adult lungs of the effects of atmospheres containing particles. The hypothesis tested in this study was that rats exposed to chronically inhaled nitrogen dioxide (NO2) or diesel exhaust during lung development were more susceptible to lung injury than rats that were exposed to these atmospheres as adults. Rats were exposed either throughout the period of lung development or as adults, and health effects in the two groups were compared at the end of exposure. Rats were exposed seven hours per day, five days per week for six months to NO2 at 9.5 ppm, to whole diesel exhaust diluted to a soot concentration of 3.5 mg/m3, or to filtered air as controls. These concentrations were selected to produce mild effects in adults. The younger group (developing) was conceived in the exposure atmospheres and exposed during gestation and through the age of six months, and the older group (adult) was exposed between six and twelve months of age. Health effects were evaluated at the end of six months' exposure and some measurements were repeated six months after the cessation of exposure. Measurements included respiratory function, pulmonary immune responses, lung clearance of radiolabeled particles, airway fluid enzymes, protein and cytology, lung tissue collagen and proteinases, lung burdens of diesel soot, lung morphometry and histopathology. Nitrogen dioxide slightly reduced body weight and altered airway fluid enzymes of both age groups, with a greater number of statistically significant differences detectable in the enzyme levels of animals exposed as adults. Normal lung development, as reflected in the size and functional efficiency at adulthood, was not affected by NO2 in this study. Diesel exhaust altered the airway fluid constituents as well as lung tissue collagen and proteinases of both age groups. Particularly striking was an almost sixfold increase in the percentage of neutrophils, a class of highly phagocytic leukocytes, in the airway fluids of adults after six months of exposure. Exhaust-exposed adults had increased numbers of cells in pulmonary lymph nodes, delayed clearance of both diesel soot and 134Cs-labeled particles, and increased lung weight. These changes did not occur in the rats exposed during development.(ABSTRACT TRUNCATED AT 400 WORDS)