The response of the endothelial permeability barrier in microvascular networks of the rat mesentery to perfused immune inflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) was examined. TNF-alpha (12.5 U/ml) treatment did not change albumin permeability, but in combination with IFN-gamma (20 U/ml), there was a marked increase in the number of sites of extravascular albumin in postcapillary venules. Endothelial integrity was characterized by cadherin-5 immunoreactivity, which was localized to the continuous intercellular junctions of endothelium in arterioles, capillaries, and venules. Perfusion with the combined cytokines showed that the increased albumin permeability was dose dependent and correlated with the focal disorganization of cadherin-5 at intercellular junctions of venular endothelium. No correlation was found between the increase in albumin permeability and the localization of intravascular leukocytes or extravascular mast cells. These results show that the combination of TNF-alpha and IFN-gamma induces an endothelial phenotype with focal loss of cadherin-5 intercellular adhesion, which, in part, facilitates passage of blood macromolecules and cells to the interstitium.