AbstractVascular endothelial growth factor-A is a major player in vascular development and a potent vascular permeability factor under physiological and pathological conditions by binding to a decoy receptor Flt1 and its primary receptor Flk1. In this study, we show that Flt1 heterozygous (Flt1+/−) mouse embryos grow up to adult without life-threatening abnormalities but exhibit a transient embryonic edema around the nuchal and back regions, which is reminiscent of increased nuchal translucency in human fetuses. Vascular permeability is enhanced and an intricate infolding of the plasma membrane and huge vesicle-like structures are seen in Flt1+/− capillary endothelial cells. Flk1 tyrosine phosphorylation is elevated in Flt1+/− embryos, but Flk1 heterozygosity does not suppress embryonic edema caused by Flt1 heterozygosity. When Flt1 mutants are crossed with Aspp1−/− mice which exhibit a transient embryonic edema with delayed formation and dysfunction of lymphatic vessels, only 5.7% of Flt1+/−; Aspp1−/− mice survive, compared to expected ratio (25%). Our results demonstrate that Flt1 heterozygosity causes a transient embryonic edema and can be a risk factor for embryonic lethality in combination with other mutations causing non-lethal vascular phenotype.