Previous studies in our laboratory showed that ANP inhibits increases in endothelial monolayer permeability to macromolecules induced by thrombin. In this present study, we investigated the second messenger system involved in the influence of ANP on monolayer permeability. In bovine aortic endothelial cells (BAEC), ANP (100 nM) caused increased cGMP levels which were measurable at 30 sec and maximal at 3 min. Addition of 8-bromo cGMP (1 mM) to BAEC monolayers mimicked the actions of ANP by inhibiting thrombin- mediated increases in permeability to [125I]-labeled bovine serum albumin. Inhibition of increases in permeability by lower concentrations of ANP was enhanced by the cGMP-selective phosphodiesterase inhibitor, M&B 22948 (100 microM). The use of ANP structural analogs which stimulate cGMP production (AP III or BNP) prevented thrombin-induced increases in monolayer permeability, whereas AP-I, which does not increase cGMP levels, was ineffective.