O(6)-Methylguanine-DNA-methyltransferase (MGMT) and nuclear factor kappaB (NF-kappaB) are two key effectors associated with the development of resistance to alkylating agent-based chemotherapy. This prompted us to hypothesize that NF-kappaB might be involved in MGMT regulation. Consistent with this hypothesis, we have discovered two putative NF-kappaB binding sites within the MGMT promoter region and showed a specific and direct interaction of NF-kappaB at each of these sites. Forced expression of the NF-kappaB subunit p65 in HEK293 cells induced an increase in MGMT expression whereas addition of the NF-kappaB super repressor DeltaNIkappaB completely abrogated the induction. We also found a significant correlation between the extent of NF-kappaB activation and MGMT expression in the glioma cell lines and the human glial tumors tested and showed that it was independent of MGMT promoter methylation. Our results are of potential clinical significance because we show that cell lines with ectopic p65 or high constitutive NF-kappaB activity are less sensitive to nitrosourea treatment and that suppression of MGMT activity with O(6)-benzylguanine completely abolishes the chemoresistance acquired by NF-kappaB. The findings of our study strongly suggest that NF-kappaB plays a major role in MGMT regulation and that MGMT is most probably the major player in NF-kappaB-mediated chemoresistance to alkylating agents.