Understanding synaptic connectivity is crucial for understanding how neuronal networks contribute to specific brain functions. However, current methods can measure the functionality only of a limited number of synaptic connections in sufficient detail. Therefore, we evaluated the applicability of a voltage imaging method in acute rat hippocampal slices to map unitary synaptic connections between CA3 neurons by using Voltron, which can detect both action potentials and small subthreshold events, such as unitary EPSPs. These connections are crucial for major hippocampal memory functions; however, controversies exist in the literature regarding the density of excitatory connections between CA3 pyramidal cells. A soma-targeted Voltron construct was sparsely expressed among CA3 neurons for 4-8 weeks, and JF549 fluorescence signals were imaged with a CMOS camera at 1 kHz in up to hundreds of neurons. We detected spontaneous spiking activity in typically 10-40 neurons. Then, we used these spikes to detect both the sub- and suprathreshold responses in other active cells (n = 2568 potential pairs). Only pairs showing clear excitatory effects in both response types were accepted as connected (n = 119 connections). We identified excitatory responses in both pyramidal cells and interneurons. As the Voltron signal persisted after fixation, the anatomical identity of cells was verified in 1103 pairs. The involvement of AMPA receptors was also confirmed by using a blocker, NBQX, and a desensitization inhibitor, cyclothiazide. Altogether, we demonstrated that Voltron imaging can effectively reveal the synaptic connectivity of neuronal networks.