The role of the endothelium in modulating cyclic nucleotide levels and intrinsic smooth muscle tone was studied in isolated rings of bovine intrapulmonary artery and vein. Cyclic 3′,5′-guanosine mono-phosphate (cGMP) levels were threefold to fourfold higher in unrubbed artery and vein than in vessels that had been denuded of endothelium. Cyclic 3,5-adenosine monophosphate (cAMP) levels were twofold higher in unrubbed than in endothelium-denuded artery, but no differences were observed in veins. Methylene blue, an inhibitor of guanylate cyclase, decreased cGMP but not cAMP levels, and this was accompanied by increases in smooth muscle tone. M&B 22,948, an inhibitor of cGMP-phosphodiesterase, increased cGMP but not cAMP levels, and this was accompanied by decreases in smooth muscle tone. Unrubbed vessels were more sensitive than endothelium-denuded vessels to the actions of both methylene blue and M&B 22,948, and this may be attributed to endothelium-depen-dent increases in cGMP turnover. Moreover, unrubbed vessels were more sensitive than endothelium-denuded vessels to contractile responses to phenylephrine and potassium, and these responses were potentiated by methylene blue and attenuated by M&B 22,948. Although indomethacin lowered cAMP levels in unrubbed artery, no changes in tone or contractile responsiveness were observed. A consistent observation was that the smaller branches of unrubbed but not endothelium-denuded intrapulmonary artery and vein had higher levels of cGMP but not cAMP, were sensitive to endotheli-um-dependent vasodilators, were more sensitive to methylene blue, and would not maintain a steady level of submaximal tone to phenylephrine when compared with larger branches from a common vascular bed. These data indicate that endothelium-derived factors in intrapulmonary artery and vein markedly influence intrinsic cGMP levels, sensitivity to endothelium-dependent vasodilators, smooth muscle tone, and contractile responsiveness.