The transcription factor NF-kappaB has recently emerged as a central regulator of the vertebrate stress response, controlling hundreds of different effector genes. We hypothesized that this transcription factor would be activated during oxygen deprivation in the anoxia-tolerant freshwater turtle, Trachemys scripta elegans. Western immunoblotting was used to examine the relative expression levels of the phosphorylated NF-kappaB inhibitor, IkappaBalpha, under normoxic and anoxic conditions in turtle tissues. Elevated levels of the phosphorylated IkappaB were found in the liver in response to 5 h of anoxia, suggesting a possible activation of the NF-kappaB pathway. Analysis of the NF-kappaB subunits, p50 and p65, showed that both mRNA transcripts and protein levels of p50 increased during anoxia, whereas p65 protein levels also increased significantly. Changes in the relative levels of the NF-kappaB subunits in the nucleus, and changes in the DNA-binding activity of NF-kappaB were also assessed and were found to be elevated in response to anoxia. Finally, reverse-transcriptase PCR was used to measure mRNA transcript levels of selected downstream target genes under NF-kappaB control and these were elevated under anoxia. These findings show that the NF-kappaB pathway is activated in turtle liver during anoxia and support a role for this transcription factor in the anoxia tolerance of T. s. elegans.