Hypoxia-inducible factor 1 (HIF-1) is a dimeric transcription factor composed of HIF-1α and HIF-1β subunits that plays an essential role in mammalian O₂homeostasis. InHif1a^−/−knockout mice, complete deficiency of HIF-1α resulted in cardiac and vascular malformations and embryonic lethality at E10.5. Between E8.75 and E9.25 striking vascular regression and abnormal remodeling occurred in the cephalic region concomitant with marked mesenchymal cell death. Similar vascular defects were observed in HIF-1α- and VEGF-deficient embryos and VEGF mRNA expression was not induced by hypoxia inHif1a^−/−embryonic stem cells. Surprisingly,Hif1a^−/−embryos demonstrated increased VEGF mRNA expression compared to wild-type embryos. In tissue culture cells, VEGF mRNA expression was induced by glucose deprivation independent of HIF-1α, providing a mechanism for increased VEGF mRNA expression inHif1a^−/−embryos, in which absence of adequate tissue perfusion resulted in both O₂and glucose deprivation. Rather than being associated with VEGF deficiency, the vascular defects inHif1a^−/−embryos were spatially correlated with cell death, the onset of which preceded vascular regression.