Naphthalene (NA) was shown to be carcinogenic, causing respiratory epithelial adenoma in the nasal cavity of male F344 rats and olfactory epithelial neuroblastoma in female F344 rats at exposure concentrations of 10-60 ppm in a 2-year inhalation study conducted by the National Toxicology Program. To explore the exposure-response relationship and threshold for nasal epithelial effects in F344 rats, a 90-day (6 h/d, 5 d/wk) inhalation study was conducted at 0, 0.1, 1, 10 and 30 ppm NA vapor. Group size for nasal cavity histopathology was 10/sex with an additional 10/sex evaluated 4 wk post-exposure. NA exposure concentrations were measured by GC/MS, and aerosol testing verified that solid NA particles were not present. There were no NA exposure-related clinical observations and mild decreases in body weight (<10%) and food/water consumption were observed primarily in the 30 ppm rats. Rat heads were cross-sectioned at six levels for microscopic examination. There were no nasal cavity lesions related to NA exposure in rats of the 0.1 ppm group. Minimal hyperplasia was observed in the transitional/respiratory epithelium of rats exposed to 1 ppm. Mild hyperplasia and minimal squamous metaplasia were observed in the respiratory epithelium of rats exposed to 10 or 30 ppm. Lesions in the olfactory epithelium were observed only in rats of the 10 or 30 ppm groups and consisted of degeneration, necrosis, areas of re-epithelialization and basal cell hyperplasia. There was remarkable recovery of effects after 4 weeks, but residual olfactory epithelial degeneration and basal cell hyperplasia were still evident.