THE endothelial cell-specific vascular endothelial growth factor (VEGF)(1-5) and its cellular receptors Flt-1 (refs 6,7) and Flk-1 (refs 8,9) have been implicated in the formation of the embryonic vasculature. This is suggested by their colocalized expression during embryogenesis(10,11) and the impaired vessel formation in Flk-1 (ref. 12) and Flt-1 (ref. 13) deficient embryos. However, because Flt-1 also binds placental growth factor(14,15), a VEGF homologue, the precise role of VEGF was unknown. Here we report that formation of blood vessels was abnormal, but not abolished, in heterozygous VEGF-deficient (VEGF(+/-)) embryos, generated by aggregation of embryonic stem (ES) cells with tetraploid embryos (T-ES)(16,17), and more impaired in homozygous VEGF-deficient (VEGF(-/-)) T-ES embryos, resulting in death at mid-gestation. Similar phenotypes were observed in F-1-VEGF(+/-) embryos, generated by germline transmission. We believe that this heterozygous lethal phenotype, which differs from the homozygous lethality in VEGF-receptor-deficient embryos, is unprecedented for a targeted autosomal gene inactivation, and is indicative of a tight dose-dependent regulation of embryonic vessel development by VEGF.