The formation of endothelial junctions and their regulation in patho-physiological processes is regulated by complex mechanisms that modulate the actomyosin based cytoskeleton and cell adhesion activities at endothelial contacts. The stability of endothelial junctions is centrally dependent on the adhesive function of VE-cadherin. We have recently shown that knock in mice expressing a fusion protein consisting of VE-cadherin and α-catenin instead of wt VE-cadherin are strongly impaired in VEGF- or histamine-induced vascular permeability and in cytokine-induced recruitment of leukocytes into various inflamed tissues. These results demonstrate that both processes require in vivo interference with the adhesive function of VE-cadherin. In vitro studies have suggested that the phosphorylation of certain tyrosine residues in VE-cadherin is involved in the induction of transendothelial permeability and transmigration of leukocytes through cultured endothelial cell layers. In agreement with this, we found that the endothelial specific receptor type protein tyrosine phosphatase, VE-PTP, associates with VE-cadherin and supports its adhesive function. We showed recently that the dissociation of VE-PTP from VE-cadherin is in vivo necessary for leukocyte extravasation and induction of vascular permeability. To analyze the relevance of tyrosine phosphorylation of VE-cadherin for the regulation of endothelial junctions in vivo we have generated knock in mice where certain tyrosine residues in VE-cadherin were replaced by phenylalanines. In addition, we generated specific antibodies for each of these different phosphorylated tyrosines within VE-cadherin. Surprisingly, we found differential regulation of the phosphorylation of these residues in vivo. Furthermore, analysis of the induction of vascular permeability and leukocyte recruitment in the different knock in mice revealed distinct roles for different tyrosine residues.