At physiological concentrations nitric oxide (NO) inhibits mitochondrial cytochrome c oxidase in competition with oxygen. We have developed a technique based on visible light spectroscopy and used it to demonstrate that endogenous NO enhances reduction of the electron transport chain, thus enabling cells to maintain their VO₂ at low oxygen concentrations (1). This favours the release of superoxide anion, which initiates the transcriptional activation of NF-κB as an early stress signalling response (2). We have also used this technique to demonstrate that NO is inactivated by cytochrome c oxidase in its oxidised state and have proposed that cessation of such inactivation at low oxygen concentrations may account for hypoxic vasodilatation (3). Many cells respond to a decrease in oxygen availability via stabilisation of hypoxia-inducible factor-1α (HIF-1α), whose accumulation is normally prevented by the action of prolyl hydroxylases. We have found that inhibition of mitochondrial respiration by low concentrations of NO leads to inhibition of HIF-1α stabilisation (4). This prevents the cell from registering hypoxia at low oxygen concentrations, which would otherwise result in upregulation of defensive genes, including those for glycolysis and angiogenesis. Furthermore, inhibition of mitochondrial respiration in hypoxia leads to redistribution of available oxygen toward non-respiratory oxygen-dependent targets (5). In addition to its interaction with cytochrome c oxidase, NO can signal for mitochondrial biogenesis via a cyclic GMP-dependent mechanism (6). Furthermore, increases in NO beyond physiological levels lead to persistent inhibition of other key enzymes in the mitochondria (7) and this may account for NO-dependent initiation of cell pathology.