Biochemical studies have indicated that the structurally simple gangliosides, including GD3 and GM3, are major glycolipid components of glioma tissues. In order to clarify the localization of the gangliosides in ethylnitrosourea-induced rat glioma, an immunohistochemical study was performed using antibodies against GM1, GM3, and GD3. The results obtained in normal fetus, newborn, and adult rat brain, and also in human glioma, were compared. In fetal and newborn rat brain, GD3 was present mainly in the neuroepithelial cell surface of the matrix and subependymal layers of the ventricular wall, but GM3 and GM1 were not detected. In adult rat brain, GD3-positive cells were absent, or present in diminished number, and GM1 was found chiefly in the neuropil of the cerebral cortex. Most of the rat glioma cells were positive for GD3, but not for GM1. It was demonstrated that the ganglioside composition of glioma cells was similar to that of immature neuroectodermal cells in fetal and newborn rat brain. Furthermore, the number of GD3-positive oligodendroglioma cells increased with tumor growth. In anaplastic gliomas and gross oligodendrogliomas, most tumor cells expressed not only GD3 but also GM3. These results suggest that GD3 is a marker of proliferating neuroectodermal cells, and that activity of the key enzymes in ganglioside synthesis alters with tumor growth and anaplastic change.