Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen and angiogenic inducer produced by a variety of cell lines and tissues. As a soluble protein that exhibits a unique target cell specificity for vascular endothelial cells, VEGF has the potential to play an important role in the development of the vascular system. To better understand the role of VEGF in the processes of vasculogenesis and embryonic angiogenesis, patterns of mRNA expression and [125I]VEGF-binding sites were examined in sections of rat embryos and surrounding tissues during the early stages of development. In situ hybridization revealed the most intense hybridization of VEGF mRNA in the giant trophoblast cells and the mesometrium of early postimplantation specimens. In contrast, only low levels of expression were detected in the embryo until later in embryonic development. Possible embryonic targets for this secreted protein, as identified by displaceable [125I] VEGF binding, were found in association with the early egg sac at E8 and evident in hemangioblasts (blood islands) within the yolk sac at E8-E11. In addition, at all stages, binding was observed along the lumina of blood vessels, of both maternal and embryonic origin. These results provide evidence to support the hypothesis that VEGF plays an important role in the normal development of the embryo and supporting tissues. In the presence of ubiquitous and persistent high affinity binding sites on vascular endothelial cells and precursors, the growth of the vascular system may be regulated in early development by regional expression of VEGF by trophoblast and maternally derived cells, and later on by cells within the embryo as they develop their differentiated phenotype.