Quantitative autoradiographic technique was applied in measuring blood-brain barrier (BBB) permeability of autochthonous gliomas in rats. In small tumors (less than 2 mm in diameter), no increase in BBB permeability was noted. As the tumor grew and neovascularization occurred, BBB permeability increased in the center of the tumor, and it was suggested that the BBB was partly disrupted in the neovascularized vessels. In the fully grown tumors, BBB permeability was markedly increased in the viable part of the tumor to levels similar to the choroid plexus. Yet, the BBB was partly preserved at the periphery of the tumor and in the brain adjacent to the tumor. The heterogeneity of the BBB phenomenon according to the stage of tumor growth may be a major obstacle for uptake of chemotherapeutic drugs that do not cross the BBB easily.