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1. Young mice are more susceptible than older mice to the virus of St. Louis encephalitis inoculated intraperitoneally, but with virus inoculated intracerebrally or intranasally, there is no significant age difference in susceptibility. The greatest change in the resistance to the virus inoculated intraperitoneally occurs between the 2nd and 3rd weeks of life. 2. The distribution of the lesions of St. Louis encephalitis in the C.N.S. of young and of old animals following intraperitoneal inoculation indicates that the virus may reach the brain either by the ascending pathway from the spinal cord or by the olfactory pathway irrespective of the age of the animal. However the ascending pathway is most frequently concerned. 3. The distribution of lesions does not offer evidence that the virus enters the C.N.S. of young animals directly from the blood stream following intraperitoneal inoculation. 4. Although widespread lesions occur earlier in the C.N.S. of young mice than in that of older mice inoculated intraperitoneally with large doses of virus, this fact is not satisfactorily explained by assuming the more rapid increase of the virus in the C.N.S. of young animals, since the latter are not more susceptible to virus inoculated directly into the brain. 5. The observations can be explained by the hypothesis that a greater amount of virus survives and reaches the portals of the C.N.S. in young animals following intraperitoneal inoculation and that this is an important factor in the influence of age on susceptibility to the virus.

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