S ubstances released by platelets during blood clotting are essential participants in events that link hemostasis and angiogenesis and ensure adequate wound healing and tissue injury repair. We assessed the participation of sphingosine 1-phosphate (Sph-1-P), a biologically active phosphorylated lipid growth factor released from activated platelets, in the regulation of endothelial monolayer barrier integrity, which is key to both angiogenesis and vascular homeostasis. Sph-1-P produced rapid, sustained, and dose-dependent increases in transmonolayer electrical resistance (TER) across both human and bovine pulmonary artery and lung microvascular endothelial cells. This substance also reversed barrier dysfunction elicited by the edemagenic agent thrombin. Sph-1-P–mediated barrier enhancement was dependent upon G_iα-receptor coupling to specific members of the endothelial differentiation gene (Edg) family of receptors (Edg-1 and Edg-3), Rho kinase and tyrosine kinase-dependent activation, and actin filament rearrangement. Sph-1-P–enhanced TER occurred in conjunction with Rac GTPase- and p21-associated kinase–dependent endothelial cortical actin assembly with recruitment of the actin filament regulatory protein, cofilin. Platelet-released Sph-1-P, linked to Rac- and Rho-dependent cytoskeletal rearrangement, may act late in angiogenesis to stabilize newly formed vessels, which often display abnormally increased vascular permeability.