Heparan sulfates, the carbohydrate chains of heparan sulfate proteoglycans, play an important role in basement membrane organization and endothelial barrier function. We explored whether endothelial cells secrete a heparan sulfate degrading heparanase under inflammatory conditions and what pathways were responsible for heparanase expression. Heparanase mRNA and protein by Western blot were induced when cultured endothelial cells were treated with cytokines, oxidized low-density lipoprotein (LDL) or fatty acids. Heparanase protein in the cell media was induced 2−10-fold when cells were treated with tumor necrosis factor α (TNFα) or interleukin 1β (IL-1β). Vascular endothelial growth factor (VEGF), in contrast, decreased heparanase secretion. Inhibitors to nuclear factor-κB (NFκB), PI3-kinase, MAP kinase, or c-jun kinase (JNK) did not affect TNFα-induced heparanase secretion. Interestingly, inhibition of caspase-8 completely abolished heparanase secretion induced by TNFα. Fatty acids also induced heparanase, and this required an Sp1 site in the heparanase promoter. Immunohistochemical analyses of cross sections of aorta showed intense staining for heparanase in the endothelium of apoE-null mice but not wild-type mice. Thus, heparanase is an inducible inflammatory gene product that may play an important role in vascular biology.