The glial envelope of dendritic spines in the visual and cerebellar cortices was evaluated by analysis of serial sections. Three-dimensional reconstructions of the protoplasmic astrocyte processes were made and the quantitative proportions of the glial cover on dendritic spines were measured and compared. Whereas dendritic spines on spiny branchlets of Purkinje cells are, with the exception of afferent axon terminals, completely covered by the glial sheath (74.44%), dendritic spines of pyramidal cells are only partially covered (28.89%), so that spine stalks and even synaptic clefts frequently lack glial isolation. A new, relatively frequent configuration of subsurface cistern--astrocyte process--dendritic spine is described. A possible functional significance of the differences in the glial ensheathment of dendritic spines in visual and cerebellar cortices is discussed.