Hypoxia-inducible factor 1 (HIF-1) is a transcription factor complex that allows induced transcription of up to 100 target genes under low oxygen availability. The cellular oxygen sensors for hypoxia dependent transcription are prolyl-hydroxylases (PHDs). Under normoxia PHDs modify the alpha subunit of the HIF-1 leading to subsequent proteasomal degradation. Under hypoxia PHD activity is reduced leading to HIF-1alpha stabilization and HIF-1 target gene expression. Because PHDs are HIF-1 target genes themselves the upregulation of PHDs under hypoxia leads to downregulation of HIF-1alpha despite the lack of oxygen. Here we investigated the role of nitric oxide (NO) in regulating hypoxia dependent gene expression. NO was found to inhibit PHD activity by an yet unknown mechanism under normoxia or in addition to hypoxia. In consequence HIF-1 alpha was stabilized and expression of the oxygen sensors PHD2 and 3 was induced by HIF-1. Induction of PHD protein levels were correlated with enhanced PHD activity and destabilization of HIF-1alpha under hypoxic conditions. Subsequent suppression of PHDs using siRNA revealed that PHD2 was exclusively responsible for regulating HIF-1alpha degradation under NO treatment. The data suggest a role for NO as an activator of the HIF-1alpha/PHD2 feedback loop under hypoxia.