Pyridine is a volatile solvent used as an intermediate in the production of insecticides, herbicides, pharmaceuticals, and dyes. Pyridine is also found in tobacco smoke. Because inhalation is a primary route of exposure to pyridine, we examined the effect of inhaled pyridine on morphology at the portal of entry, the nose. Nasal tissues from F344/N rats exposed using a nose-only mode 6 hr/day for 4 days to either filtered air (controls) or one of two concentrations of pyridine vapor were examined histologically. The rats had been killed 18 hr after the last exposure. The two pyridine concentrations were the current threshold limit value (TLV, 5 ppm) and a high concentration (444 ppm). Olfactory epithelial lesions in rats exposed to both concentrations of pyridine included vacuolar degeneration of sustentacular cells; focal, marked attenuation of the epithelium; loss of vacuolar degeneration of sustentacular cells; focal, marked attenuation of the epithelium; loss of neurons; and the presence of intraepithelial luminal structures. The lesions were only slightly more severe in the rats exposed to 444 ppm compared to those rats exposed to 5 ppm pyridine. The results show that inhalation of pyridine at the current TLV concentration of 5 or 444 ppm causes lesions in the olfactory epithelium of rats.