Hypoxia-induced muscle wasting has been observed in several environmental and pathological conditions. However, the molecular mechanisms behind this loss of muscle mass are far from being completely elucidated, certainly in vivo. When studying the regulation of muscle mass by environmental hypoxia, many confounding factors have to be taken into account, such as decreased protein ingestion, sleep deprivation or reduced physical activity, which make difficult to know whether hypoxia per se causes a reduction in muscle mass. Aim: We hypothesized that acute exposure to normobaric hypoxia (11% O2) would repress the activation of the mTOR pathway usually observed after a meal and would activate the proteolytic pathways in skeletal muscle. Methods: Fifteen subjects were exposed passively for 4h to normoxic and hypoxic conditions in a random order after consumption of a light breakfast. A muscle biopsy and a blood sample were taken before, after 1h and 4h of exposure. Results: After 4h, plasma insulin concentration and the phosphorylation state of PKB and S6K1 in skeletal muscle were higher in hypoxia than in normoxia (p