Morphologically a synapse consists of a presynaptic release site containing vesicles, a postsynaptic element with membrane specialization, and a synaptic cleft between them. The number of release sites shapes the properties of synaptic transmission between neurons. Although excitatory interactions between cortical neurons have been examined, the number of release sites remains unknown. We have now recorded excitatory postsynaptic potentials evoked by single pyramidal cells in hippocampal interneurons and visualized both cells using biocytin injections. Light and electron microscopy showed that excitatory postsynaptic potentials were mediated by a single synapse. We also reconstructed the entire axon arborization of single pyramidal cells, filled in vivo, in sections counterstained for parvalbumin, which selectively marks basket and axo-axonic cells. Single synaptic contacts between pyramidal cells and parvalbumin-containing neurons were dominant (> 80%), providing evidence for high convergence and divergence in hippocampal networks.