Introduction The experimental lesion produced by cerebrospinal fluid exchange provides an unusual opportunity to investigate demyelination and remyelination in the adult mammalian neuraxis. The lesion was discovered empirically when animals subjected to repeated withdrawal and reinjection of cerebrospinal fluid (CSF) were observed to develop conspicuous neurological deficits.1Though the mechanism by which this lesion is produced remains unknown, the unique occurrence of remyelination following demyelination has provided impetus for light and electron microscopic studies1-3and for this autoradiographic analysis. Taken together, these studies indicate that adult glial cells have a degree of versatility not hitherto recognized. To produce the lesion nothing is added to the CSF; it is simply withdrawn and reinjected repeatedly through a needle in the cisterna magna. A portion of animals so treated (∼40%) develop partial paralysis of the posterior extremities; the more severely affected are unable to walk. At autopsy the lesion is