The regulation of endothelial permeability is poorly understood. An increase in endothelial permeability in the pulmonary microvasculature, however, is critical in noncardiogenic pulmonary edema and other diffuse inflammatory reactions. In the present study thrombin and Escherichia coli hemolysin (HlyA), a membrane-perturbing bacterial exotoxin, were used to alter hydraulic permeability of porcine pulmonary artery and human endothelial cell monolayers. We also investigated the pharmacological approach of adenylyl cyclase activation/phosphodiesterase (PDE) inhibition to block endothelial hyperpermeability. Thrombin (1–5 units/ml) and H1yA (0.5–3 hemolytic units/ml) dose and time dependently (>15 min) increased endothelial permeability. Forskolin, cholera toxin, and prostaglandin E₁, which all stimulate adenylyl cyclase activity, abrogated this effect. One MM dibutyryl CAMP, a cell membrane-permeable CAMP analogue, was similarly active. Endothelial hyperpermeability was also reduced dose dependently by inhibitors of different PDE isoenzymes (motapizone, rolipram, and zardaverine, which block PDE3 and/or PDE4). The effectiveness of PDE inhibitors was increased in the presence of adenylyl cyclase activators. Analysis of cyclic nucleotide hydrolyzing PDE activity in lysates of human umbilical vein endothelial cells showed high activities of PDE isoenzymes 2, 3, and 4. Consistent with the functional data PDE3 and PDE4 were the major cAMP hydrolysis enzymes in intact endothelial cells. We conclude that the hyperpermeability of pulmonary endothelial monolayers, evoked by thrombin or H1yA, can be blocked by the simultaneous activation of adenylyl cyclase and inhibition of PDEs, especially of PDE3 and PDE4. The demonstration of PDE isoenzymes 2–4 in human endothelial cells will help optimize this therapeutic approach.