An electron microscopic examination was done on 8 samples of cerebrospinal fluid (CSF) from Icelandic sheep infected by the intracerebral route with visna virus. The specimens were collected 1 month, 2 months, and 4 years after infection. A differentail cell count done on low-power electron micrographs showed that the cellular exudate was composed of mononuclear cells mainly macrophages and lymphocytes with a few plasma cells. Macrophages were with one exception more numerous than lymphocytes and an increased proportion of macrophages showed evidence of phagocytosis with time after infection. Reactive lymphocytes were in general more numerous than small lymphocytes. Various stages in the maturation of plasma cells were observed. The cellular composition in the CSF is compatible with the view that visna is an immunopathological process. Myelin figures and fragments of myelinated axons were observed in two specimens indicating an active myelin-breakdown. The possibility that escape of myelin into the CSF may lead to sensitization to myelin antigens and perpetuation of this chronic neurologic affection is discussed. Visna virions could not be demonstrated.