Organotypic cultures from newborn rat brains of different postnatal ages were maintained in an apparently healthy condition for at least two months. It is concluded that damage to myelination in vitro is due to direct glial cell damage. Although radiation damage to blood vessels in vivo may initiate or contribute to myelination damage the present findings show that glial injury in absent of blood vessels leads to myelination damage at doses comparable to those used in vivo.