Neuropilin 1 (NP-1) is a receptor for vascular endothelial growth factor (VEGF) 165 (VEGF₁₆₅) and acts as a coreceptor that enhances VEGF₁₆₅ function through tyrosine kinase VEGF receptor 2 (VEGFR-2). Transgenic overexpression of np-1results in an excess of capillaries and blood vessels and a malformed heart. Thus, NP-1 may have a key role in vascular development. However, how NP-1 regulates vascular development is not well understood. This study demonstrates how NP-1 can regulate vasculogenesis and angiogenesis in vitro and in vivo. In homozygous np-1mutant (np-1^−/−) murine embryos, vascular sprouting was impaired in the central nervous system and pericardium. Para-aortic splanchnopleural mesoderm (P-Sp) explants fromnp-1^−/− mice also had vascular defects in vitro. A monomer of soluble NP-1 (NP-1 tagged with Flag epitope) inhibited vascular development in cultured wild-type P-Sp explants by sequestering VEGF₁₆₅. In contrast, a dimer of soluble NP-1 (NP-1 fused with the Fc part of human IgG) enhanced vascular development in cultured wild-type P-Sp explants. Moreover, the NP-1–Fc rescued the defective vascular development in culturednp-1^−/− P-Sp explants. A low dose of VEGF alone did not promote phosphorylation of VEGFR-2 on endothelial cells from np-1^−/− embryos, but simultaneous addition of a low dose of VEGF and NP-1–Fc phosphorylated VEGFR-2 significantly. Moreover, NP-1–Fc rescued the defective vascularity of np-1^−/− embryos in vivo. These results suggest that a dimer form of soluble NP-1 delivers VEGF₁₆₅ to VEGFR-2–positive endothelial cells and promotes angiogenesis.