Newborn mice were inoculated with a murine papovavirus, K virus, by intracranial, intraperitoneal, oral, and intranasal routes, and the pathogenesis of infection was studied with immunofluorescence, virus assay, and histopathology. Inoculation by each route produced a fatal interstitial pneumonia. Pulmonary vascular endothelium and, to a lesser extent, cells lining hepatic sinusoids were the major sites of viral replication, but intranuclear antigen or inclusions or both were also found in extrapulmonary vascular endothelia, spleens, lymph nodes, and brains. Although K virus produced a predominantly respiratory illness, the virus was less infectious by intranasal than by oral inoculation and did not replicate in respiratory epithelial tissues. The earliest site of K virus replication was the jejunal submucosa, suggesting that in nature K virus may be transmitted by the oral route. Viral antigen was present in brains of animals inoculated by each route and correlated with the duration of viremia. Despite the presence of abundant viral antigen, however, the nervous system remained morphologically normal. The present study indicates that a member of the papovavirus group may produce clinically silent, noninflammatory involvement of the central nervous system during the initial infection of its natural host.