During angiogenesis, endothelial cells undergo proliferation, reorganization, and stabilization to establish a mature vascular network. This process is critical for establishing a functional circulatory system during development and contributes to the pathological process of tumor growth. Here we report that embryos deficient for the ERK5 MAPK die between embryonic days 10.5 and 11.5 with angiogenic failure and cardiovascular defects. We show that ERK5 deficiency leads to an increased expression of the vascular endothelial growth factor (VEGF), dysregulation of which has been shown to impede angiogenic remodeling and vascular stabilization. Our data also reveal that ERK5 negatively regulates transcription from the vegf locus during hypoxic responses. Importantly, ERK5 is required at an earlier developmental stage than p38␣, and p38␣ does not compensate for ERK5 deficiency. These results demonstrate that ERK5 plays a specific role in the regulation of early angiogenesis.Vasculogenesis and angiogenesis are two critical processes of establishing the cardiovascular system in vertebrate embryos. Vasculogenesis involves generation of the primitive vascular plexus from angioblasts that have recently differentiated from mesodermal tissues. This process leads to the formation of the early heart tube and major blood vessels such as the dorsal aorta (reviewed in Refs.