Beginning with the earliest investigations of California encephalitis (CE) virus by Hammon et al., the rabbit has been implicated as an important wild reservoir for the virus. Viremia considered as probably sufficient to infect mosquitoes developed in domestic rabbits without manifest illness after they had been infected in the laboratory with the original, prototype BFS-283 strain. Significant levels of neutralizing antibody to the infecting virus resulted. Serologic surveys of mammals in California, the Rocky Mountain areas of the northern United States and southern Canada, and in Wisconsin, Ohio, and Florida, have all suggested that the rabbit or hare is frequently involved in the natural chain of transmission of viruses in the CE group. In Ontario and Czechoslovakia the domestic rabbit has been successfully used as a sentinel or indicator of CE-group virus activity in nature. A series of laboratory-infection experiments demonstrated that in domestic rabbits infected with Tahyna virus, viremic titers developed that were sufficient to infect mosquitoes.