Objective and Design: The effects of lipopolysaccharide (LPS), a potent inflammatory mediator, on the shear stress stimulated release of adenosine triphosphate (ATP) were investigated on endothelial cells from human umbilical vein in primary culture.¶Methods: Human umbilical vein endothelial cells (HUVEC) in primary cultures were subjected to shear stress using a cone and plate apparatus. ATP released by the cells was measured by luminometry, using a luciferin-luciferase assay.¶Results: Under conditions of shear stress alone (25 dyn/cm²), ATP accumulates into the culture medium and reaches a maximum after 3 to 5 min of stimulation (121.7 ± 13.2 pmol/ml). The shear stress-stimulated release of ATP was significantly increased after a 4  h pre-incubation of endothelial cells with 50 μg/ml (314.4 ± 26.7 pmol/ml) and 10 μg/ml lipopolysaccharide (207.7 ± 22.2 pmol/ml). Dexamethasone, an anti-inflammatory glucocorticoid, inhibited the effects of lipopolysaccharide.¶Conclusions: These results show that non-damaged endothelial cells release ATP under experimental inflammatory conditions and support an early role of extracellular ATP in the inflammatory process.