Continued in vitro cultivation in a Maitland type medium resulted in a marked modification of the extraneural pathogenicity of the Venezuelan equine encephalomyelitis virus. The ability of the virus to induce lethal infections after peripheral inoculation was almost completely lost for mice 42 or more days of age, was somewhat reduced for mice 28 days of age, but was still retained for mice 21 or less days of age. The virulence of the virus by the cerebral route remained essentially unaffected for mice of any of the experimental age groups. Prolonged cultivation also resulted in almost complete attenuation of the virus for rabbits and guinea pigs by the intraperitoneal route.