Objective: ROS - reactive oxygen species - are generated by the mitochondrial respiratory chain during hypoxia in the cell or develop in the course of oxidative stress as in ischemic diseases, inflammation or cancer. The main source of ROS within the cell is complex III of the mitochondrial respiratory chain. Whereas high concentrations of ROS lead to damage of the cell, ROS in low concentrations are part of cellular signaling pathways. Considering the short half life of ROS most likely H2O2 and derived ROS act as signaling molecules. One pathway influenced by ROS is cellular oxygen sensing. Cellular oxygen sensing is mediated by the heterodimeric transcription factor Hypoxia- inducible-Factor 1 (HIF-1) and its regulating enzyme Prolyl-Hydroxylase 2 (PHD2). HIF consists of the two constitutively expressed subunits HIF- 1a and HIF-1b. Under normoxia HIF-1a is hydroxylated by PHD2 and thus marked for proteasomal degradation. Hypoxic conditions inhibit PHD2 activity, HIF- 1a dimerizes with HIF-1b to form active HIF-1. This leads to hypoxic gene expression. Up to date controversial results for the impact of ROS on HIF-1 and PHD2 have been found. Therefore we studied the effect of H2O2 on HIF- 1a and PHD2 in a human osteosarcoma cells (U2OS) and other cell lines. Methods and Results: Western blot analysis revealed that H2O2 first stabilized but later decreased HIF- 1a whereas PHD2 was constantly reduced. Luciferase reporter gene assays and Western blot analysis showed that reduction of the HIF target gene PHD2 was neither due to lowered HIF-1 transcriptional activity nor to increased proteasomal degradation. PHD2 activity was reduced by H2O2.Conclusion: H2O2 and its derived ROS have a biphasic effect on cellular oxygen sensing. Reduction of PHD2 activity may lead to the initial increase in HIF-1a. The later decrease in HIF-1a is still unresolved. To elucidate the exact way how ROS influence cellular oxygen sensing further investigations are needed.