Little is known about the expression of nuclear factor kappa B (NF-kappaB) in oligodendrocytes. We demonstrate that under normal culture conditions only 21% of oligodendrocytes express NF-kappaB p65; exposure to hypoxia increased this to 69%. A similar increase in total p65 was seen with Western blots. Northern blot analysis demonstrated that NF-kappaB p65 mRNA increased during the first few h following hypoxia, suggesting that exposure to hypoxia affects NF-kappaB p65 transcription or p65 mRNA stability. Under normoxic conditions only 2% of the oligodendrocytes exhibited activated NF-kappaB; this was increased to 8% by hypoxia followed by reoxygenation. Despite activation of NF-kappaB, no detectable expression of the pro-inflammatory genes iNOS and ICAM-1 was seen. This is the first demonstration that hypoxia can up-regulate p65.