Intermittent hypoxia (IH) occurs in many pathophysiological conditions. The molecular mechanisms associated with IH, however, received little attention. Previous studies have reported that c-fos via formation of activator protein-1 (AP-1) transcription factor contributes to adaptive responses to continuous hypoxia. In the present study, using a cell culture model we examined whether IH activates c-fos and AP-1 and if so, by what mechanisms. Experiments were performed on rat pheochromocytoma cells exposed to 21% O₂ (normoxia) or 60 and 120 cycles of IH, each cycle consisting 15 seconds of hypoxia followed by 4 min of normoxia. IH resulted in a significant elevation of c-fos mRNA as well as transcriptional activation. IH was more potent and induced a longer lasting activation of c-fos than comparable, cumulative duration of continuous hypoxia. IH increased AP-1 activity and tyrosine hydroxylase (TH) mRNA, an AP-1 regulated downstream gene, and these effects were prevented by anti-sense c-fos. Superoxide dismutase mimetic, a potent scavenger of superoxide anion, prevented IH-induced c-fos, AP-1 and TH activations. IH increased superoxide anion levels in mitochondria as evidenced by decreased aconitase enzyme activity and increased hydrogen peroxide levels, a stable dismutated product of superoxide anion. Complex I of the mitochondrial electron transport chain was markedly inhibited in IH exposed cells. Pharmacological inhibitors of complex I mimicked the effects of IH during normoxia and occluded the effects of IH on c-fos activation, suggesting the involvement of mitochondria electron transport chain in the generation of superoxide anion during IH. These results suggest IH-induced c-fos mediated transcriptional activation involves oxidative stress.