Solo astroglial cultures have randomly distributed, intramembranous, orthogonal arrays of particles (assemblies) which are only revealed by freeze-fracture electron microscopy. Co-culturing astrocytes with brain endothelial cells brought about localized, tightly packed assembly aggregates and greatly increased the overall assembly density. Cytosol homogenates of freeze-thawed brain endothelial cells caused a transient increase in astroglial assembly numbers. These results, taken together with the fact that astrocytes in vivo have the highest concentration of perivascular sites, suggest that brain endothelial cells influence the distribution and concentration of astrogial assemblies both in vivo and in vitro through cellular interactions. Meningeal cells and fibroblasts also augmented the astroglial assembly densities in co-culture, while neuronal cells (cerebellar granule cells and PC12 cells primed with nerve growth factor) and other control cell types did not affect assembly number in co-culture with astrocytes. Moreover, brain endothelial cells did not induce any formation of assemblies in the membranes of two transformed astroglial cell lines.