Summary: We investigated and modeled the effect of Ca$^{2+}$ on presynaptic facilitation. Facilitation of transmitter release from a presynaptic terminal is believed to arise from the local increase of Ca$^{2+}$ concentration. We measured and simulated the effect of Ca$^{2+}$ on facilitation at the lobster neuromuscular synapse. Facilitation was measured using quantal analysis of postsynaptic potentials recorded focally during trains of stimuli. We found a facilitation component, which was not affected by Ca$^{2+}$ concentration. Transmitter release was simulated using a model of two pools of presynaptic vesicles available for release. The simulation reproduced quantitatively Ca$^{2+}$-dependent and Ca$^{2+}$-independent components of facilitation.