Carbon-fiber microelectrochemical methods were utilized in this study to measure individual exocytotic events of secretion of serotonin and histamine from washed rabbit platelets. The quantal release of serotonin was quantitatively characterized with a delta-granule serotonin concentration of 0.6 M and secretion time course of 7 ms. Additionally, extracellular osmolarity influences quantal size, causing quantal size increases under hypotonic conditions, presumably due to the influx of cytosolic serotonin into the halo region of the delta-granules.