Trains of action potentials evoked rises in presynaptic Ca2+ concentration ([Ca2+]i) at the squid giant synapse. These increases in [Ca2+]i were spatially nonuniform during the trains, but rapidly equilibrated after the trains and slowly declined over hundreds of seconds. The trains also elicited synaptic depression and augmentation, both of which developed during stimulation and declined within a few seconds afterward. Microinjection of the Ca2+ buffer EGTA into presynaptic terminals had no effect on transmitter release or synaptic depression. However, EGTA injection effectively blocked both the persistent Ca2+ signals and augmentation. These results suggest that transmitter release is triggered by a large, brief, and sharply localized rise in [Ca2+]i, while augmentation is produced by a smaller, slower, and more diffuse rise in [Ca2+]i.