Effects of leptomeningeal tumor on the brain parenchyma was studied by the immunohistochemical method with astroprotein (GFAP) and tubulin in a rat model of meningeal carcinomatosis. Thickening of subpial glial lining (external glial layer) and hypertrophy of subpial astrocytes, detected by the antiserum to GFAP, was the early sign of parenchymal involvement. The glial lining was continuous as far as the tumor cells were confined to the subarachnoid space, however, penetration of tumor cells into subpial brain was associated with disruption of the glial lining. Speculative role of this lining in preventing the tumor cell to infiltrate into brain tissue was discussed. In contrast to the prominent immunohistochemical changes in astrocytes, neuronal tubulin immunoreactivity was not altered even in the late stage of the disease. The present study demonstrated that the leptomeningeal dissemination of tumor cells did cause pathologic change in brain parenchyma as was evidenced by the reactive change of astrocytes. However, the preserved immunoreaction for tubulin suggested that the nerve cell damage was not severe even at the advanced stage of the disease.