Neuropeptide Y (NPY), a 36-aa peptide, is widely distributed in the brain and peripheral tissues. Whereas physiological roles of NPY as a hormone/neurotransmitter have been well studied, little is known about its other peripheral functions. Here, we report that NPY acts as a potent angiogenic factor in vivo using the mouse corneal micropocket and the chick chorioallantoic membrane (CAM) assays. Unlike vascular endothelial growth factor (VEGF), microvessels induced by NPY had distinct vascular tree-like structures showing vasodilation. This angiogenic pattern was similar to that induced by fibroblast growth factor-2, and the angiogenic response was dose-dependent. In the developing chick embryo, NPY stimulated vascular sprouting from preexisting blood vessels. When [Leu³¹Pro³⁴]NPY, a NPY-based analogue lacking high affinity for the NPY Y₂ receptor but capable of stimulating both Y₁ and Y₅ receptors, was used in the corneal model, no angiogenic response could be detected. In addition, NPY failed to induce angiogenesis in Y₂ receptor-null mice, suggesting that this NPY receptor subtype was mediating the angiogenic signal. In support of this finding, the Y₂ receptor, but not Y₁, Y₄, or Y₅ receptors, was found to be widely expressed in newly formed blood vessels. Further, a delay of skin wound healing with reduced neovascularization was found in Y₂ receptor-null mice. These data demonstrate that NPY may play an important role in the regulation of angiogenesis and angiogenesis-dependent tissue repair.