As the cells forming the luminal vascular surface, endothelium is exposed to alterations in the vascular microenvironment, such as hypoxaemia. In this report we demonstrate that hypoxia, with pO2 as low as 12-14 mmHg, was not toxic to endothelium, but reversibly modulated central cellular functions essential for maintenance of homeostasis: permeability of monolayers to solutes increased in a dose-dependent manner, and cell surface coagulant properties were shifted to promote activation of coagulation. The anticoagulant cofactor thrombomodulin was suppressed and an apparently novel activator of factor X, distinct from the classical extrinsic and intrinsic systems, was induced. The hypoxia-induced factor X activator was cell surface-associated, had properties of a cysteine protease, had Mr corresponding to approximately 100 kDa, based on sodium dodecyl sulphate-polyacrylamide electrophoresis (SDS-PAGE), and isoelectric point (pI) approximately 5.0. These findings indicate that hypoxia dynamically modulates endothelial function providing insights into the contribution of microvascular endothelial dysfunction in the pathogenesis of vascular lesions.