Expression of two different types of intermediate filaments, vimentin filaments and glial filaments, was studied immunohistochemically in experimental rat gliomas. Although vimentin filaments are most commonly seen in mesenchymal cells, recent immunocytochemical study demonstrated that this type of filaments can be recognized also in glial cells during early cell differentiation and in tumor cells of epithelial origin. In the present communication, distribution of vimentin filaments in rat glial tumors was investigated and compared with that of glial filaments by using specific antiserum to each protein subunit, vimentin and astroprotein (GFAP). Ethylnitrosourea (50 mg/kg) was injected subcutaneously into 3 day-old Wistar rats. After four to ten months, brains of animals were removed, fixed in 95% ethanol and embedded in paraffin. Peroxidase-antiperoxidase method was carried out on 6 micron-thick sections. In normal portion of the brain, immunoreaction for vimentin was noted in ependymal cells and in vascular endothelial cells but not in astrocytes. This distribution contrasted with that of astroprotein (GFAP), which distributed in astrocytes but not in normal ependymal cells. These findings confirmed that the two antisera used in the present study do not crossreact to each other. In contrast to the absence of vimentin immunoreaction in normal astrocytes, a number of tumor cells showed positive reaction to the antiserum to vimentin. Mixed glioma with astrocytoma and oligodendroglioma had both astroprotein (GFAP)-positive and negative cells. Well developed cellular processes were noted in astroprotein (GFAP)-positive cells (astrocytoma cells). Weak immunoreaction for vimentin was noted in those cells.(ABSTRACT TRUNCATED AT 250 WORDS)