Co‐expression of NRP1 and (VEGFR‐2) KDR on the surface of endothelial cells (EC) enhances VEGF₁₆₅ binding to KDR and EC chemotaxis in response to VEGF₁₆₅. Overexpression of NRP1 by prostate tumor cells in vivo results in increased tumor angiogenesis and growth. We investigated the molecular mechanisms underlying NRP1‐mediated angiogenesis by analyzing the association of NRP1 and KDR. An intracellular complex containing NRP1 and KDR was immunoprecipitated from EC by anti‐NRP1 antibodies only in the presence of VEGF₁₆₅. In contrast, VEGF₁₂₁, which does not bind to NRP1, did not support complex formation. Complexes containing VEGF₁₆₅, NRP1, and KDR were also formed in an intercellular fashion by co‐culture of EC expressing KDR only, with cells expressing NRP1 only, for example, breast carcinoma cells. VEGF₁₆₅ also mediated the binding of a soluble NRP1 dimer to cells expressing KDR only, confirming the formation of such complexes. Furthermore, the formation of complexes containing KDR and NRP1 markedly increased ¹²⁵I‐VEGF₁₆₅ binding to KDR. Our results suggest that formation of a ternary complex of VEGF₁₆₅, KDR, and NRP1 potentiates VEGF₁₆₅ binding to KDR. These complexes are formed on the surface of EC and in a juxtacrine manner via association of tumor cell NRP1 and EC KDR. J. Cell. Biochem. 85: 357–368, 2002. © 2002 Wiley‐Liss, Inc.