Peroxidase-antiperoxidase (PAP) immunohistochemical staining, utilizing a specific antibody to the glial fibrillary acidic protein (GFA), was employed to analyze gliogenesis in the central nervous system of rhesus monkeys ranging in age from embryonic day 38 (E38) to birth (E165) and through the second postnatal month. All major subdivisions of the brain contain glial cells, recognized by the presence of dark brown horseradish peroxidase (HRP) reaction product. Neuronal elements are not stained with this immunocytochemical technique. The first class of glial cells to appear during development are the radial glial cells; the radial fibers fan out from the ventricular and subventricular zones, where their cell bodies reside, to the pial surface where they terminate with conical endfeet. These glial cells appear within the first third of gestation, being present in the spinal cord and brainstem by E41; in the diencephalon by E45; and in the telencephalon and cerebellum by E47. The next class of glia to appear is the Bergmann glial cell of the cerebellar cortex, which can be stained by E54. Bergmann glial cells located below the Purkinje cell layer issue parallel processes which extend up to the pial surface. Within each major subdivision of the brain, massive numbers of elongated glial fibers continually alter their distinctive patterns to maintain constant ventricular-pial surface relationships during the major tectogenetic changes which occur throughout embryonic development. In Nissl-counterstained sections columns of migrating neurons are observed juxtaposed to GFA-positive radial and Bergmann glial fibers. Radial glial cells assume a variety of transitional forms during the process of their transformation into maturea astrocytes. This transformation occurs in each structure at specific embryonic ages and is initiated after neuronal migration has begun to subside. The number of astroglial cells increases at an accelerated pace after neurogenesis is complete. The immunohistochemical localization of radial fibers at relatively early stages of embryonic development indicates that glial cells are present concomitantly with neurons, raising the possibility that at least two distinct populations of cell precursors compose the proliferative zones. Furthermore, the demonstration of large numbers of radial glial cells in all brain regions during the peak of neuronal migration and a close structural relationship between elongated glial fibers and migrating neurons support the concept that glia play a significant role in the guidance and compartmentalization of neuronal elements during development.