To understand the cellular basis of learning and memory, the neurophysiology of the hippocampus has been largely examined in thin transverse slice preparations. However, the synaptic architecture along the longitudinal septo-temporal axis perpendicular to the transverse projections in CA1 is largely unknown, despite its potential significance for understanding the information processing carried out by the hippocampus. Here, using a battery of powerful techniques, including 3D digital holography and focal glutamate uncaging, voltage-sensitive dye, two-photon imaging, electrophysiology, and immunohistochemistry, we show that CA1 pyramidal neurons are connected to one another in an associational and well-organized fashion along the longitudinal axis of the hippocampus. Such CA1 longitudinal connections mediate reliable signal transfer among the pyramidal cells and express significant synaptic plasticity. These results illustrate a need to reconceptualize hippocampal CA1 network function to include not only processing in the transverse plane, but also operations made possible by the longitudinal network. Our data will thus provide an essential basis for future computational modeling studies on information processing operations carried out in the full 3D hippocampal network that underlies its complex cognitive functions.