Sensing and responding to changes in oxygen partial pressure ensures the cellular oxygen supply to be tightly controlled to balance the risks of oxidative damage vs. oxygen deficiency. The hypoxia inducible factor (HIF) regulatory system comprises prolyl hydroxylases (PHDs), the von Hippel Lindau protein (pVHL) as well as the 26S-proteasome and transmits changes in oxygenation into adequate adaptive responses. A functional HIF-response requires stabilization of the a-subunit, e.g. HIF-1a?under hypoxia and dimerization with HIF-1a to drive target gene activation. Intriguingly, nitric oxide (NO) shares with hypoxia the ability to stabilize HIF-1a and to mimic hypoxic responses under normoxia. Mechanistically, NO blocks PHD activity and attenuates proline hydroxylation of HIF-1a with subsequent accumulation of the protein. However, under hypoxia NO facilitates destruction of HIF-1a and thus reverses HIF signaling. Under these conditions, NO impairs respiration and avoids oxygen gradients that limit PHD activity. An additional layer of regulatory complexity is added to the system by the interaction of NO with O2- with the notion that hypoxia may evoke O2- formation. Signaling qualities attributed to NO are antagonized by compensatory flux rates of O2- and vice versa to adjust levels of HIF-1a under normoxia and hypoxia. The importance of NO and O2- in affecting target proteins, i.e. HIF-1a can be relevant to many (patho)physiological circumstances.