Platelet adhesion, activation and aggregation are essential for primary haemostasis, but are also critically involved in the development of acute arterial thrombotic occlusion. Stimulation of the collagen receptor glyocoprotein VI (GPVI) leads to PLC-dependent IP₃ production with subsequent platelet activation, due to increased intracellular calcium concentration ([Ca²⁺]_i). Although it has been described that tricyclic antidepressants potentially impair platelet activation, nothing is hitherto known about potential effects of the tricyclic antidepressant doxepin on platelet Ca²⁺ signaling and thrombus formation. As shown in the present study doxepin significantly diminished the stimulatory effect of GPVI agonist collagen-related peptide (CRP) on intracellular Ca²⁺ release as well as subsequent extracellular Ca²⁺ influx. Doxepin was partially effective by impairment of CRP-dependent inositol triphosphate (IP₃) production. Moreover, doxepin abrogated CRP-induced platelet degranulation, integrin α_IIbβ₃ activation and aggregation. Finally, doxepin markedly blunted in vitro platelet adhesion to collagen and thrombus formation under high arterial shear rates (1700^-sec). In conclusion, doxepin is a powerful inhibitor of GPVI-dependent platelet Ca²⁺ signaling, platelet activation and thrombus formation.