Glioblastoma is very rarely found outside the central nervous system. The ability of rat C6 glioblastoma cells to intravasate into central nervous system and pial blood vessels is tested using a rat homografting model and two in vitro models. In vivo, scanning electron microscopy demonstrates that upon grafting C6 cells into implantation pockets in rat cortex, blood vessels can be spared in large digestion cysts formed in host brain parenchyma. Immunocytochemistry of the grafted rat cortex reveals that the glioblastoma cells are upon the blood vessel basement membrane, surrounded by the extracellular matrix material, fibronectin. The endothelial cells of the blood vessel are inside the laminin and fibronectin, and there were areas of endothelial cell hyperplasia. C6 cells are not observed inside blood vessels. In vitro, C6 cell cultures seeded with blood vessels from fresh rat pia exhibit the same relationship of the C6 glioblastoma cells to the blood vessel as those in the other models. The C6 cells migrate upon the pial blood vessel basement membrane but do not intravasate into the blood vessel. To ascertain whether structure and components of the blood vessel basement membrane are important factors in glioblastoma cell exclusion from blood vessels, C6 cells are seeded upon artificial basement membrane hydrated gel wafers. C6 cells migrate into the artificial basement membrane gel wafer by 1 day after seeding. These data indicate that glioblastoma cells are confined to the central nervous system by an inability to pass through vital basement membrane.