Synaptic transmission produced by artificially generated presynaptic depolarizations, having the same waveform as normal presynaptic action potentials, was studied in the squid giant synapse. These stimulated spikes produced synaptic transmission virtually indistinguishable from that obtained with the original presynaptic spike activation. The technique allows a direct determination of the onset and amplitude of the Ca current, ICa, triggered by the presynaptic action potential and the relationship between ICa and postsynaptic response. The results closely resemble theoretical predictions by a previously published model for synaptic transmission in this synapse and demonstrate that ICa occurs at the falling phase of the presynaptic spike.