In the lung, hypoxia induces pulmonary hypertension caused by vasoconstriction and vascular remodeling. Additionally, hypoxia is an inducer of angiogenesis, which is assumed to counteract pulmonary hypertension. We asked whether the anti-angiogenic factor endostatin--a cleavage product of collagen XVIII--participates in the vascular alterations induced by hypoxia. By employing Western blotting of tissue extracts of murine brain, liver and heart an endostatin fragment of 22 kDa was detectable, whereas in lung and aorta additional bands of 24 and 26 kDa were found. The amount of these larger fragments was increased in tissues obtained from mice housed for 4 days or 3 weeks at hypobaric hypoxia. By immunohistochemistry endostatin was detected in association with elastic fibers and in close neighborhood to smooth muscle cells of intrapulmonary vessels and the aorta. In the lung, the activity of matrix metalloproteinases (MMP) known to generate endostatin by cleavage of collagen XVIII was increased (MMP-2) and decreased (proMMP-9), respectively, by hypoxia. Elevated amounts of endostatin within the aortic wall of mice exposed to hypobaric hypoxia may stabilize the vascular wall by inhibition of microvascular sprouting. The surprising finding of increased endostatin in the lung presumably contributes to the development of pulmonary hypertension by reduction of angiogenesis.