During hypoxia, a cellular adaptive response activates hypoxia-inducible factors (HIFs; HIF-1 and HIF-2) that respond to low tissue-oxygen levels and induce the expression of a number of genes that promote angiogenesis, energymetabolism, and cell survival.HIF-1 andHIF-2 regulate endothelial cell (EC) adaptation by activating genesignaling cascades that promote endothelialmigration, growth, and differentiation. AnHIF-1 to HIF-2 transition or switch governs this process from acute to prolonged hypoxia. In the present study, we evaluated the mechanisms governing theHIF switch in 10 different primaryhuman ECs fromdifferent vascular beds during the early stages of hypoxia. The studies demonstrate that the switch fromHIF-1 to HIF-2 constitutes a universalmechanismof cellular adaptation to hypoxic stress and that HIF1A and HIF2A mRNA stability differences contribute to HIF switch. Furthermore, using 4 genome-widemRNAexpression arrays ofHUVECs during normoxia and after 2, 8, and 16 h of hypoxia, we show using bioinformatics analyses that, although a number of genes appeared to be regulated exclusively by HIF-1 or HIF-2, the largest number of genes appeared to be regulated by both.