1. Voltage clamp studies were performed in squid giant synapse after blockage of the voltage-dependent sodium and potassium conductances. 2. Presynaptic depolarization under these conditions demonstrates the presence of voltage-dependent calcium conductance change for the duration of the voltage step, and a tail current at the break of the pulse. 2. This calcium current triggers a postsynaptic response which can be measured directly at the postsynaptic fiber. 4. These voltage clamp experiments have allowed the development of a mathematical model that describes the kinetics of the calcium current and the relationship between calcium current and transmitter release.