Recent studies using retroviral labeling of subventricular zone (SVZ) progenitors in vivo in neonatal rats have directly demonstrated the generation of both astrocytes and oligodendrocytes from these progenitors. In the present study, we used a recombinant retroviral vector encoding beta-galactosidase, and analyzed brains within the first week after retroviral injection to trace the early routes that SVZ cells take as they migrate into white matter and cortex and characterized the early morphological and antigenic changes that accompanied their differentiation. SVZ cells follow specifically definable migratory routes as they colonize the cortex and subcortical white matter. Glial progenitors do not populate the cortex in a systematic, laminar fashion, as do neuroblasts. The abundance of labeled progenitors in radial arrangements and the close apposition of many immature cells to vimentin+ radial glial processes, suggest that glial progenitors migrate along radial glia. Labeled SVZ cells, which displayed a simple, unipolar or bipolar morphology, lacked detectable vimentin and nestin intermediate filaments. Similarly, beta- galactosidase-positive cells in white matter lacked these filaments. In contrast, labeled, multipolar cells in the cortex, and a few of the immature-appearing cortical cells expressed nestin and vimentin. At these early time points, GFAP was not detected in beta-galactosidase- labeled cells. Multipolar cells in cortex frequently displayed processes extending toward and contacting blood vessels. These observations suggest that the expression of nestin and vimentin occurs after progenitors emigrate from the SVZ and that filament expression and contact with blood vessels represent an early stage of astrocyte differentiation.