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Cells exposed to low oxygen concentrations respond by initiating defense mechanisms, including the stabilization of hypoxia-inducible factor (HIF) 1α, a transcription factor that upregulates genes such as those involved in glycolysis and angiogenesis. Nitric oxide and other inhibitors of mitochondrial respiration prevent the stabilization of HIF1α during hypoxia. In studies of cultured cells, we show that this effect is a result of an increase in prolyl hydroxylase–dependent degradation of HIF1α. With the use of Renilla luciferase to detect intracellular oxygen concentrations, we also demonstrate that, upon inhibition of mitochondrial respiration in hypoxia, oxygen is redistributed toward nonrespiratory oxygen-dependent targets such as prolyl hydroxylases so that they do not register hypoxia. Thus, the signaling consequences of hypoxia may be profoundly modified by nitric oxide.