Abstract: VISNA, a slow demyelinating infection of the central nervous system of sheep was described in an earlier publication (Sigurdsson, Palsson, and Grimsson, 1957). This disease begins with pleocytosis in the cerebro-spinal fluid (c.s.f.) from 1 to 3 months after intracerebral inoculation of active material and develops extremely slowly. The pleocytosis may persist for a few weeks or as long as 2 years or more. A considerable proportion of sheep so affected later develop progressive paralysis which may appear as late as 1 or 2 years after the inoculation and leads to death. In other cases pleocytosis in the c.s.f. regresses and the animal recovers without ever showing external signs of the infection. The characteristic lesion in the brain and spinal cord is in the white matter and in many cases its nature and extent is reminiscent of the lesions seen in the human demyelinating diseases. Studies on Visna have continued and the methods employed are the same as described by Sigurdsson, Pa6lsson, and Grimsson (1957) unless otherwise stated.