Atherosclerosis has an important inflammatory component. Macrophages accumulating in atherosclerotic arteries produce prostaglandin E(2) (PGE(2)), a main inflammatory mediator. Platelets express inhibitory receptors (EP(2), EP(4)) and a stimulatory receptor (EP(3)) for this prostanoid. Recently, it has been reported in ApoE(-/-) mice that PGE(2) accumulating in inflammatory atherosclerotic lesions might contribute to atherothrombosis after plaque rupture by activating platelet EP(3), and EP(3) blockade has been proposed to be a promising new approach in anti-thrombotic therapy. The aim of our investigation was to study the role of PGE(2) in human atherosclerotic plaques on human platelet function and thrombus formation. Plaque PGE(2) might either activate or inhibit platelets depending on stimulation of either EP(3) or EP(4), respectively. We found that the two EP(3)-antagonists AE5-599 (300 nM) and AE3-240 (300 nM) specifically and completely inhibited the synergistic effect of the EP(3)-agonist sulprostone on U46619-induced platelet aggregation in blood. However, these two EP(3)-antagonists neither inhibited atherosclerotic plaque-induced platelet aggregation, GPIIb/IIIa exposure, dense and alpha granule secretion in blood nor reduced plaque-induced platelet thrombus formation under arterial flow. The EP(4)-antagonist AE3-208 (1-3 μM) potentiated in combination with PGE(2) (1 μM) ADP-induced aggregation, demonstrating that PGE(2) enhances platelet aggregation when the inhibitory EP(4)-receptor is inactivated. However, plaque-induced platelet aggregation was not augmented after platelet pre-treatment with AE3-208, indicating that plaque PGE(2) does not stimulate the EP(4)-receptor. We found that PGE(2) was present in plaques only at very low levels (15 pg PGE(2)/mg plaque). We conclude that PGE(2) in human atherosclerotic lesions does not modulate (i.e. stimulate or inhibit) atherothrombosis in blood after plaque rupture.