LPA increases transcellular resistance across cultured rabbit corneal epithelial and endothelial cell monolayers, and the effect is mediated through the LPA(1) receptor and signaled through Galpha(i/o). S1P-stimulated increases in endothelial resistance are also signaled through Galpha(i/o). Both LPA and S1P prevented increased transcellular permeabilities induced by PMA, and increased actin stress fiber formation in epithelial and endothelial cells.