Morphometric techniques were used to examine cellular and tissue changes occurring in male and female rat lungs exposed to ozone for a prolonged time. F344/N rats were exposed to 0.0, 0.12, 0.5, or 1.0 parts per million (ppm)* ozone for six hours per day, five days per week, for 20 months. Changes in cell volume, cell surface ratios, and cellular characteristics were studied in the terminal bronchioles and in the proximal alveolar regions of the lungs. Animals exposed for 20 months to 0.5 or 1.0 ppm ozone demonstrated dramatic increases in the volume of interstitium and epithelium along the alveolar ducts. The thickening of the epithelium was caused by an epithelial metaplasia in which the normal squamous epithelium was modified to a cuboidal epithelium similar, but not identical, to the type found in terminal bronchioles. This bronchiolar epithelial metaplasia was directly related to dose of ozone, and was characterized by differentiated ciliated cells and Clara cells similar to those found in terminal bronchioles; undifferentiated cuboidal cells also were found in the animals exposed to 0.5 and 1.0 ppm ozone. A mild fibrotic response was seen in the animals exposed to 1.0 ppm ozone, with increases in both the interstitial matrix and cellular interstitium. The individual components of the interstitial matrix, including collagen, elastin, basement membrane, and acellular spaces, all were increased. The increase in cellular interstitium was due to an increase in the volume of interstitial fibroblasts. A slight inflammatory response, identified by an increase in alveolar macrophages, was observed in the animals exposed to 1.0 ppm. The terminal bronchioles were less affected than the proximal alveolar region by the ozone exposures, which may indicate a resistance of this tissue to ozone damage. The changes in the terminal bronchioles mainly consisted of a shift in cell type from ciliated to Clara cells in the animals exposed to 1.0 ppm ozone. The bronchiolar epithelial metaplasia observed in the proximal alveolar ducts may indicate that a protective mechanism develops in response to prolonged exposure to high concentrations of ozone.