—Endothelial cell migration is necessary for the formation of new blood vessels. We investigated the effects of 2 lysophospholipid mediators, sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA), on endothelial cell migration. S1P and LPA stimulated migration of fetal bovine heart endothelial cells (FBHEs) in a 3D-modified Boyden chamber assay with concentrations as low as 15 nmol/L stimulating a 2-fold change and concentrations in the 1- to 2-μmol/L range stimulating 14- to 20-fold changes. S1P specifically stimulated the migration of several endothelial cell strains but did not stimulate the migration of tumor cells or smooth muscle cells. LPA stimulated some endothelial and nonendothelial cell types to migrate. For FBHEs, S1P and LPA were mostly chemokinetic in checkerboard assays. S1P and LPA stimulated extracellular signal–regulated kinase 1/2 phosphorylation and enhanced paxillin localization to focal contacts, with no discernible change in the actin cytoskeleton in FBHEs. To characterize responsible receptor-dependent signaling pathways, we investigated the involvement of G_i, Rho, and phosphoinositide 3-OH kinase in S1P- and LPA-stimulated migration. Although perturbation of all 3 signaling molecules resulted in decreased migration, the mechanisms underlying the decreased migration were different. Pertussis toxin treatment, to target G_i, caused endothelial cells to develop dense bundles of F-actin and distribute paxillin staining to the cell periphery in response to S1P or LPA. Modification of Rho with C3 toxin disrupted the actin cytoskeleton. Inhibition of phosphoinositide 3-OH kinase decreased S1P- or LPA-induced endothelial cell migration with only minor disruption of the actin cytoskeleton. Inhibition of extracellular signal-regulated kinase kinase with PD98059 caused a loss of phosphorylation of extracellular signal–regulated kinase 1/2, similar to pertussis toxin, but only a minimal decrease in migration. These results indicate that S1P and, for some cells, LPA stimulate migration of endothelial cells through a mechanism that likely requires a balance between G_i and Rho signaling to achieve the cytoskeletal remodeling necessary for cell migration.