Recurrent gliomas are most often treated by chemotherapy. However, these tumors typically acquire resistance to most drugs administered, and patients will usually die of recurrent tumor. Factors which may play a role include overexpression of putative multidrug resistance genes, such as the multidrug resistance gene 1 (MDR1), multidrug resistance associated protein gene (MRP), 06-alkylguanine, DNA alkyltransferase gene (06MT) and excision repair cross complementing gene 1 (ERCC1). Tumor hypoxia has also been shown to be associated with drug resistance in other soft tissue tumors. Since gliomas have regions of diminished oxygenation, and have clinical resistance to chemotherapy, the relationship between phenotypic resistance to chemotherapy after hypoxic exposure and expression of drug resistance genes was investigated in glioma cell lines (U373 MG, PFAT-MT). After a 24 hour exposure to hypoxia, drugs 1, 3-bis, 2-chloroethyl-1-nitrosurea (BCNU) and cis-diammine, dichloroplatinum II (CDDP) were administered, and cell survival was determined. Hypoxic exposure was associated with increased survival of the cell lines after administration of BCNU and CDDP, with resistance to BCNU 15 to 30-fold when compared to cells which did not undergo hypoxic exposure. Both tumor cell lines also showed some degree of resistance to CDDP, although not to the extent of BCNU (2 to 3-fold increased resistance). The expression of the drug resistance genes was found to be unchanged when comparing cells which had undergone hypoxic exposure and those which had not. Thus, hypoxic exposure is associated with substantial drug resistance in brain tumor cell lines. The lack of correlation between the induced phenotype and known drug resistance genes suggests other mechanisms may be acting in these tumors in hypoxic conditions.