Depolarization of the neuronal soma augments synaptic output onto postsynaptic neurons via long-range, axonal cable properties. Here, we report that the range of this somatic influence is spatially restricted by not only axonal path length but also a branching-dependent decrease in axon diameter. Cell-attached recordings of action potentials (APs) from multiple axon branches of a rat hippocampal CA3 pyramidal cell revealed that an AP was broadened following a 20 mV depolarization of the soma and reverted to a normal width during propagation down the axon. The narrowing of the AP depended on the distance traveled by the AP and on the number of axon branch points through which the AP passed. These findings were confirmed by optical imaging of AP-induced calcium elevations in presynaptic boutons, suggesting that the somatic membrane potential modifies synaptic outputs near the soma but not long-projection outputs. Consistent with this prediction, whole-cell recordings from synaptically connected neurons revealed that depolarization of presynaptic CA3 pyramidal cells facilitated synaptic transmission to nearby CA3 pyramidal cells, but not to distant pyramidal cells in CA3 or CA1. Therefore, axonal geometry enables the differential modulation of synaptic output depending on target location.