Under hypoxic conditions, cells suppress energy-intensive mRNA translation by modulating the mammalian target of rapamycin (mTOR) and pancreatic eIF2alpha kinase (PERK) pathways. Much is known about hypoxic inhibition of mTOR activity; however, the cellular processes activating PERK remain unclear. Since hypoxia is known to increase intracellular reactive oxygen species (ROS), we hypothesized that hypoxic ROS regulate mTOR and PERK to control mRNA translation and cell survival. Our data indicate that although exogenous ROS inhibit mTOR, eIF2alpha, and eEF2, mTOR and eEF2 were largely refractory to ROS generated under moderate hypoxia (0.5% O(2)). In direct contrast, the PERK/eIF2alpha/ATF4 integrated stress response (ISR) was activated by hypoxic ROS and contributed to global protein synthesis inhibition and adaptive ATF4-mediated gene expression. The ISR as well as exogenous growth factors were critical for cell viability during extended hypoxia, since ISR inhibition decreased the viability of cells deprived of O(2) and growth factors. Collectively, our data support an important role for ROS in hypoxic cell survival. Under conditions of moderate hypoxia, ROS induce the ISR, thereby promoting energy and redox homeostasis and enhancing cellular survival.