A sequential study of lesions of the nasal cavity associated with sialodacryoadenitis virus (SDAV) infection was made in the laboratory rat. Wistar rats were intranasally inoculated with approximately 10(3) TCID50 of the coronavirus SDAV. Transverse sections of four regions of the nasal cavity from inoculated and control animals were examined by light microscopy and immunohistochemistry at 2, 4, 6, 8, 10, and 14 days postinoculation (PI). Lesions were observed in the following regions of the upper respiratory tract: respiratory epithelium, transitional epithelium, olfactory epithelium, nasolacrimal duct, vomeronasal organ, and the submucosal glands of the nasal passages. In general, in structures lined by ciliated epithelial cells, there was focal to segmental necrosis with exfoliation of affected cells and polymorphonuclear cell infiltration during the acute stages, progressing to squamous metaplasia during the reparative stages. Repair in these regions was essentially complete by 14 days PI. In the olfactory epithelium and the vomeronasal organ, there was interstitial edema with necrosis and exfoliation of epithelial cells and minimal to moderate inflammatory cell response during the acute stages. Residual reparative lesions were still evident in the olfactory epithelium, the columnar epithelium and neuroepithelium of the vomeronasal organ, and the nasolacrimal duct at 14 days PI. Viral antigen was demonstrated by immunohistochemistry in all regions during the acute stages of the disease, with the exception of the vomeronasal organ. In view of these findings, infections of the respiratory tract with viruses such as SDAV could have significant effects on functions such as olfaction and chemoreception for > or = 2 weeks postexposure in this species.