We investigated the role of surface adhesion molecules on endothelial cells in regulating vascular permeability, in vitro and in vivo. Cultured rat endothelial cells (REC) express Thy-1, intercellular adhesion molecule-1 (ICAM-1), CD44 and RT1A. Permeability of albumin across the REC monolayer increased through the interaction of Thy-1 and anti-Thy-1 mAb, but not through ICAM-1 and anti-ICAM-1, CD44 and anti-CD44, and RT1A and anti-RT1A mAb. This anti-Thy-1-effect was completely inhibited when the calmodulin antagonist W-7 and the protein kinase inhibitor H-7 was combined, while the IL-6-mediated increase in REC permeability was blocked by either W-7 or H-7, independently. The anti-Thy-1-mediated permeability increase was additively augmented when IL-6 was admixed. These data suggest that intracellular signaling pathways of anti-Thy-1- and IL-6-mediated permeability regulation may be overlapping to some extent but are largely independent. As anti-Thy-1-treatment generated rearrangement of vimentin filaments within REC, alteration of the cytoskeleton distribution may possibly correlate with the regulation of permeability. Although Thy-1-expression on rat vascular endothelium in vivo was not evident, it was induced at sites of Freund's complete adjuvant-induced dermatitis. The administered anti-Thy-1 mAb exclusively located on vascular endothelial surface at the sites of inflammation. Vascular permeability in inflamed skin tissues was significantly augmented when anti-Thy-1 but not anti-ICAM-1, anti-CD44 or anti-RT1A mAb was administered i.v., without affecting populations of inflammatory cells. The collective evidence suggests that Thy-1 induced on rat endothelium is one important regulatory event in vascular permeability at sites of inflammation.