Platelet secretion not only drives thrombosis and haemostasis, but also mediates a variety of other physiological and pathological processes. The ubiquitous SNARE machinery and a number of accessory proteins have been implicated in regulating secretion in platelets. Although several platelet SNAREs have been identified, further members of the SNARE family may help fine-tune platelet secretion. In this study I identified expression of t-SNAREs VTIlA, VTIlB (Qb SNAREs) and STX8 (Qc SNARE) in human and mouse platelets. Those 'novel' SNAREs were able to interact with each other and previously reported SNAREs VAMP8 (R-SNARE) and STXll (Qa SNARE), thus suggesting existence of a secondary SNARE complex in addition to the widely accepted SNAP23-VAMP8- STXll complex. In following mouse studies, whereas neither gene was found to be essential for a -granule or lysosome secretion, Stx8-/- platelets showed a significant defect in dense granule secretion and aggregation, that was most pronounced at intermediate concentrations of agonists. Addition of exogenous ADP could rescue the aggregation defect but failed to restore dense granule secretion, suggesting the defect lies in the 'primary' secretory pathway. Pretreatment with P2Y receptors antagonists reduced secretion and aggregation to the same extent in WT and Stx8-/- platelets, suggesting that the ADP-driven positive feedback mechanism was not defective in Stx8-/- platelets. In addition, STX8 was found to play a role in regulating pro-coagulant activity suggesting novel roles for SNAREs in platelets. Neither Vtila-/- nor Vtilb-/- showed any significant defects, suggesting complementarity between those homologues in platelets. STX8 therefore specifically contributes to dense granule secretion and represents another member of a growing family of genes that play distinct roles in differentially regulating granule release from platelets. Taken together, data presented in this Thesis not only provides first evidence of an additional SNARE complex present in platelets but also suggests novel roles for SNAREs in regulation of thrombosis and haemostasis.