Dissociated glial and neuronal precursors from the spinal cord or cerebral cortex of late rat embryos are cultured on top of glass coverslips to ascertain the in vitro effects of high glucocorticoid levels on the process of myelination and on astrocyte markers during postnatal development and determine the dependence of those effects on glucocorticoid receptor activation. The study includes the immunohistochemical processing of culture-carrying coverslips with antibodies to astrocytic, neuronal, oligodendroglial, and myelin proteins to determine changes in those markers after glucocorticoid treatment. In addition, we describe oligodendrocyte-astrocyte cultures from neonatal embryos to determine the effects of high glucocorticoids on the morphology of oligodendrocytes in the absence of developing neurons.