The size, geometry and composition of the extracellular space (ECS) play an important role in influencing the biological behavior of primary brain tumors. Experiments employing the real-time TMA iontophoretic method to determine the size and geometry of the ECS, by monitoring the diffusion of TMA ions in the ECS, revealed a dramatic increase in ECS size in brain neoplasms when compared with that of unaffected brain cortex. Further, the increase of ECS volume in tumors was shown to correlate with increasing proliferative activity and increasing cellularity of astrocytomas. The increase in ECS size was surprisingly accompanied by a significant increase in diffusion barriers, slowing the diffusion of molecules in the ECS of tumors. In low-grade tumors, diffusion is hindered by the presence of a dense net of tumor cell processes. In high-grade gliomas, in which the cellular processes are shortened with reduced branching, the increase in diffusion barriers is caused by the overproduction of specific components of the extracellular matrix (ECM) by the tumor cells, mainly tenascin. The ECM glycoproteins produced represent a substrate for the subsequent adhesion and migration of tumor cells through the enlarged ECS. However, they might also critically reduce the diffusion of therapeutics into the tumor. The presence of tenascin in the ECS of a neoplasm correlates significantly with the increased malignancy of the tumor and a poor clinical outcome of the disease, thus making the immunohistochemical detection of tenascin diagnostically useful as a prognostic marker and a marker of aggressive biological behavior of tumors.