A fatal encephalitis develops in 7-day-old albino rats following nasal instillation or intracerebral injn. of Japanese B encephalitis virus. The virus produces a uniformly fatal infection in 7- and 8-day-old rats when injd. intraperit. While some 12-day-old rats are susceptible to the virus when inoculated either by the intracerebral, intraperit. or intranasal route they are more resistant than 7- and 8-day-old rats. Rats 21 and 27 days of age appear to be resistant to the virus, since when it is injected intra-cerebrally into animals of this age it does not induce a clinically apparent infection.