Previous studies have demonstrated that cGMP and cAMP reduce the endothelial permeability for fluids and macromolecules when the endothelial permeability is increased by thrombin. In this study, we have investigated the mechanism by which cGMP improves the endothelial barrier function and examined whether nitric oxide (NO) can serve as an endogenous modulator of endothelial barrier function. Thrombin increased the passage of macromolecules through human umbilical vein and human aortic endothelial cell monolayers and concomitantly increased [Ca]²⁺ in vitro. Inhibition of these increases by the intracellular Ca²⁺ chelator BAPTA indicated that cytoplasmic Ca²⁺ elevation contributes to the thrombin-induced increase in endothelial permeability. The cGMP-dependent protein kinase activators 8-bromo-cGMP (8-Br-cGMP) and 8-(4-chlorophenylthio)cGMP (8-PCPT-cGMP) decreased the thrombin-induced passage of macromolecules. Two pathways accounted for this observation. Activation of cGMP-dependent protein kinase by 8-PCPT-cGMP decreased the accumulation of cytoplasmic Ca²⁺ in aortic endothelial cells and hence reduced the thrombin-induced increase in permeability. On the other hand, in umbilical vein endothelial cells, cGMP-inhibited phosphodiesterase (PDE III) activity was mainly responsible for the cGMP-dependent reduction of endothelial permeability. The PDE III inhibitors Indolidan (LY195115) and SKF94120 decreased the thrombin-induced increase in permeability by 50% in these cells. Thrombin treatment increased cGMP formation in the majority of, but not all, cell cultures. Inhibition of NO production by N^G-nitro-l-arginine methyl ester (L-NAME) enhanced the thrombin-induced increase in permeability, which was restricted to those cell cultures that displayed an increased cGMP formation after addition of thrombin. Simultaneous elevation of the endothelial cGMP concentration by atrial natriuretic factor, sodium nitroprusside, or 8-Br-cGMP prevented the additional increase in permeability induced by L-NAME. These data indicate that cGMP reduces thrombin-induced endothelial permeability by inhibition of the thrombin-induced Ca²⁺ accumulation and/or by inhibition of cAMP degradation by PDE III. The relative contribution of these mechanisms differs in aortic and umbilical vein endothelial cells. NO can act in vitro as an endogenous permeability-counteracting agent by raising cGMP in endothelial cells of large vessels.