The Sakai strain of Getah virus isolated from a feverish horse was examined for pathogenicity for mice. Its pathogenicity depended on the age of the host and the route of inoculation. Intracerebral inoculation caused death to mice less than 9 days old. It induced retarded growth, insufficient hair growth, paralysis and sporadic death in mice 11 days old; no symptoms were observed in mice 13 days old. The virus multiplied rapidly and extensively in every tested organ, particularly in the muscular tissue, of the suckling mouse. The virus was transmitted horizontally among the litter-mates and vertically from the dam to her offspring via milk.