Comparison of cytoskeletal preparations obtained from newborn and adult rat brain showed similar patterns on SDS-PAGE. However, coelectrophoresis of the newborn and adult preparations revealed distinct differences in the mobility of 2 major bands in the molecular weight range of 50--70 000. In adult brain cytoskeletons, the main band in the 50 000 range co-migrated with purified rat GFA protein (apparent molecular weight 53 000). No major band co-migrated with purified rat vimentin (apparent molecular weight 57 000). The reverse was true for newborn brain cytoskeleton. In adult and newborn brain cytoskeleton a major band co-migrated with the 150 000 neurofilament polypeptide isolated from rat spinal cord by immunoaffinity chromatography. Another neurofilament polypeptide (apparent molecular weight 72 000) was prominent in adult but not in newborn brain cytoskeleton. Conversely, newborn brain cytoskeleton comprised a band trailing behind the 72 000 neurofilament polypeptide. This band was not present in adult brain cytoskeleton. The distribution of vimentin in newborn rat brain was studied by immunofluorescence microscopy and compared to the distribution of GFA protein. As previously reported, a relatively limited number of GFA positive cells are present in the brain at this stage compared to later in development. Conversely, the large number of vimentin positive cells in newborn brain was well in keeping with the presence of a prominent vimentin band in cytoskeletal preparations obtained from this tissue. With the exception of meninges and blood vessels, vimentin appeared to be mainly localized in immature glia: periventricular glia; glia in non-myelinated white matter; radial glia in cerebral cortex and basal ganglia; Bergmann glia in cerebellum (Bergmann glia are still GFA negative in newborn rat). The neuroblastic germinal layers in hippocampus and cerebellum did not stain with vimentin antisera.