Although ozone (O3)-induced bronchiolitis has been morphologically characterized, effects of O3 on the upper respiratory tract have not been thoroughly investigated. The purpose of this study was to determine whether exposures to ambient levels of O3 induce lesions in the nasal mucosa. Bonnet monkeys were exposed to 0.00, 0.15, or 0.30 ppm O3 for 6 or 90 days, 8 hours/day. After exposure, nasal mucosa was processed for light and electron microscopy. Quantitative changes were evident in the nasal transitional and respiratory epithelium. At 6 or 90 days of exposure to 0.15 or 0.30 ppm O3 lesions consisted of ciliated cell necrosis, shortened cilia, and secretory cell hyperplasia. Inflammatory cell influx was only present at 6 days of exposure. Ultrastructural changes in goblet cells were evident at 90 days. Ambient levels of O3 can induce significant nasal epithelial lesions, which may compromise upper respiratory defense mechanisms.