Angiotensinogen shares with other members of the serine protease inhibitor (serpin) family antiangiogenic properties. Angiotensinogen inhibits in vitro endothelial cell proliferation, and is antiangiogenic in ovo in the chick chorioallantoic membrane assay. The cellular mode of action of angiotensinogen has been studied by applying purified human angiotensinogen or Chinese hamster ovary cells producing recombinant angiotensinogen onto the developing chorioallantoic membrane. Vessel density of the control and angiotensinogen-treated areas was quantitated by using Sambucus nigra lectin, a specific endothelial cell marker. After 48 h of angiotensinogen treatment by either applying purified angiotensinogen or angiotensinogen-producing Chinese hamster ovary cells, there was a 70% decrease in mesodermic vessel density in comparison to the control sections. Angiotensinogen treatment induced a strong decrease in endothelial cell proliferation of the chorioallantoic membrane vasculature, as shown by incorporation of bromo-deoxyuridine. Two days after local angiotensinogen treatment, increased apoptosis of endothelial cells of mesodermal blood vessels was detected by transferase-mediated deoxyuridine triphosphate nick end labeling assay. As assessed by in situ hybridization, the gene expression pattern of the main vascular growth factors and their receptors was not altered by angiotensinogen. Angiotensinogen, therefore, impairs angiogenesis without altering the expression level of vascular growth factors through the induction of apoptosis and decreased endothelial cell proliferation.