The present study was aimed to clarify the pathogenesis of Sendai virus infection to the central nervous system (CNS) of mice. One-to 2-day-old suckling and 4-week-old mice were inoculated intracerebrally with the virus. The virus multiplied higher in suclings than in adults. Immunofluorescent studies in sucklings revealed that the viral antigens appeared initially in ependyma, choroid plexus epithelium, and meninges. Subsequently they spread to subependymal cells and finally were found in neurons of hippocampus for as long as 4 months postinfection. In adults, however, the viral antigens rapidly disappeared in the early stage. Most mice inoculated intracerebrally with Sendai virus appeared healthy, although hydropcephalus developed in a few mice. Virus-specific antibody and interferon production seemed to have no influence on the persistent infection of Sendai virus in the CNS of mice. One of the most significant findings may be that the viral antigens persist in the brain for as long as 4 months in a latent form. This may offer a useful model for the study of latent CNS infection of paramyxoviruses.