NEUROTRANSMITTER release is mediated by Ca2+ dependent exocytosis of synaptic vesicles’, Neither the amount of transmitter released from individual synaptic vesicles nor the kinetics of this process have yet been directly determined, Using carbon fibres as electrochemical detectors(2,3), we have measured release of the neurotransmitter serotonin from cultured neurons of the leech(4). This technique allowed us to monitor transmitter discharge from single synaptic vesicles as spike-like oxidation currents at high time resolution, providing new insight into the mechanism of neuronal exocytosis, Two types of signals were characterized, corresponding to exocytosis of small clear and large dense core vesicles present in these cells, A small vesicle discharges about 4,700 transmitter molecules with a time constant in the region of 260 mu s, whereas large vesicles release their content of approximately 80,000 molecules with a time constant of about 1.3 ms, Release from both vesicle types is initiated rapidly, with a rise time of less than 60 mu s, suggesting an abrupt opening of a preassembled fusion pore.