electrochemical measurement of endogenous serotonin release from human blood platelets platelet aggregation in the bloodstream is tightly associated with the secretory function of platelets based on several types of cytoplasmic secretory granules each sequestering distinct chemical messenger species densebody granules are one prominent type of secretory granule responsible for storing small molecule chemical messengers upon platelet activation the timely and rapid release of these small molecules is critical in facilitating platelet aggregation therefore techniques capable of measuring realtime granule content release are needed to understand the fundamental properties of platelet secretion and aggregation existing techniques lack adequate time resolution or require potentially toxic exogenous reagents for realtime measurement of granule content release herein we demonstrate a labelfree electrochemical method based on the endogenous electroactive chemical messenger serotonin 5hydroxytryptamine or 5ht for the realtime measurement of densebody granule secretion from platelet suspensions fastscan cyclic voltammetry fscv using carbonfiber microelectrodes was chosen on the basis of its excellent temporal resolution high sensitivity and the ability to provide the electrochemical signature cyclic voltammograms for molecular identification realtime serotonin release from thrombinstimulated human platelet suspensions was successfully measured and the amount and time course of the bulk serotonin release were found to agree well with data obtained from single platelet measurements thus confirming accurate characterization of granular secretion furthermore this electrochemical method was applied to study the stimulationsecretion coupling in platelets serotonin storage and release dynamics with applied pharmacological agents and chemical messenger storage deficiency in hermanskypudlak syndrome hps platelets and the potential of this method to reveal secretion behavior in both normal and diseased platelets has clearly been demonstrated