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T he essential contribution of the antidepressant-sensitive serotonin (5-HT) transporter SERT (which is encoded by the SLC6A4 gene) to platelet 5-HT stores suggests an important role of this transporter in platelet function. Here, using SERT-deficient mice, we have established a role for constitutive SERT expression in efficient ADP- and thrombin-triggered platelet aggregation. Additionally, using pharmacological blockers of SERT and the vesicular monoamine transporter (VMAT), we have identified a role for ongoing 5-HT release and SERT activity in efficient human platelet aggregation. We have also demonstrated that fibrinogen, an activator of integrin αIIbβ3, enhances SERT activity in human platelets and that integrin αIIbβ3 interacts directly with the C terminus of SERT. Consistent with these findings, knockout mice lacking integrin β3 displayed diminished platelet SERT activity. Conversely, HEK293 cells engineered to express human SERT and an activated form of integrin β3 exhibited enhanced SERT function that coincided with elevated SERT surface expression. Our results support an unsuspected role of αIIbβ3/SERT associations as well as αIIbβ3 activation in control of SERT activity in vivo that may have broad implications for hyperserotonemia, cardiovascular disorders, and autism.