The localization of glycogen phosphorylase in gerbil brain was determined by immunoelectron microscopy using the pre-embedding peroxidase technique. Electron-dense granular reaction product of peroxidase activity was observed in astrocytes of all brain regions examined (cerebral cortex, striatum, cerebellar cortex, hippocampal formation, corpus callosum, mesencephalic trigeminal nucleus). The reaction product was distributed in a diffuse pattern throughout the cytoplasmic matrix of perikarya and processes; sometimes the nucleus of labeled astrocytes also contains immunopositive material. The light microscopically visible glycogen phosphorylase immunoreactivity associated with capillaries could be characterized as a staining of astrocytic endfeet ensheathing capillaries. Endothelial cells and pericytes were never labeled. In addition to astrocytes, ependymal cells also presented immunopositive material in their cytoplasm. On the other hand, no reaction product was observed in cells identified as oligodendroglia or microglia. Neurons (with the exception of neurons of the mesencephalic trigeminal nucleus), their processes, and their synaptic endings were free of reaction product. In the neuropil we frequently observed immunopositive glial processes adjacent to synaptic structures. This intimate spatial relationship may be interpreted as a morphological sign of a metabolic interaction. The data support the hypothesis that astroglia play a key role in glycogen metabolism and energization of the brain.