The peripheral susceptibility of hamsters was compared with the peripheral and intracerebral susceptibility of mice by using the AT₃₁ strain of Japanese encephalitis virus, which has a strong peripheral infectivity. The results obtained are summarized as follows.
1) In hamsters inoculated intracardiacally with 300 ic LD₅₀ for mice of the virus, the virus appeared in the blood 12 hours after inoculation (a. i.), reached the highest level (10^2.68PFU/ml) 2 days a. i. Then it decreased to a certain level where it remained up to 4 days a. i. In mice inoculated intravenously, the virus reached the highest level (10^1.91PFU/ml) 1 day a. i. and decreased after that to disappear from the blood 4 days a. i. Neutralizing antibody appeared in the blood of both hamsters and mice 3 days a. i.
2) Hamsters showed a very high peripheral susceptibility. In them, infection was established by inoculation even with a dose less than 1 LD₅₀ for mice, and hemagglutination-inhibition antibody of high titer was produced. Almost all the hamsters inoculated were involved in latent infection, presenting a good contrast to the inoculated mice which exhibited a high fatality rate.
3) The typical histological changes of encephalitis in the brains were observed in almost all the hamsters infected.