Action potentials in central neurons are initiated near the axon initial segment, propagate into the axon, and finally invade the presynaptic terminals, where they trigger transmitter release. Voltage-gated Na+ channels are key determinants of excitability, but Na+ channel density and properties in axons and presynaptic terminals of cortical neurons have not been examined yet. In hippocampal mossy fiber boutons, which emerge from parent axons en passant, Na+ channels are very abundant, with an estimated number of ~2000 channels per bouton. Presynaptic Na+ channels show faster inactivation kinetics than somatic channels, suggesting differences between subcellular compartments of the same cell. Computational analysis of action potential propagation in axon-multibouton structures reveals that Na+ channels in boutons preferentially amplify the presynaptic action potential and enhance Ca2+ inflow, whereas Na+ channels in axons control the reliability and speed of propagation. Thus, presynaptic and axonal Na+ channels contribute differentially to mossy fiber synaptic transmission.