Rationale: Cardiovascular health depends on proper development and integrity of blood vessels. Ets variant 2 (Etv2), a member of the E26 transforming–specific family of transcription factors, is essential to initiate a transcriptional program leading to vascular morphogenesis in early mouse embryos. However, endothelial expression of the Etv2 gene ceases at midgestation; therefore, vascular development past this stage must continue independent of Etv2. Objective: To identify molecular mechanisms underlying transcriptional regulation of vascular morphogenesis and homeostasis in the absence of Etv2. Methods and Results: Using loss- and gain-of-function strategies and a series of molecular techniques, we identify Friend leukemia integration 1 ( Fli1 ), another E26 transforming–specific family transcription factor, as a downstream target of Etv2. We demonstrate that Etv2 binds to conserved Ets-binding sites within the promoter region of the Fli1 gene and governs Fli1 expression. Importantly, in the absence of Etv2 at midgestation, binding of Etv2 at Ets-binding sites in the Fli1 promoter is replaced by Fli1 protein itself, sustaining expression of Fli1 as well as selective Etv2-regulated endothelial genes to promote endothelial cell survival and vascular integrity. Consistent with this, we report that Fli1 binds to the conserved Ets-binding sites within promoter and enhancer regions of other Etv2-regulated endothelial genes, including Tie2 , to control their expression at and beyond midgestation. Conclusions: We have identified a novel positive feed-forward regulatory loop in which Etv2 activates expression of genes involved in vasculogenesis, including Fli1 . Once the program is activated in early embryos, Fli1 then takes over to sustain the process in the absence of Etv2.