We provide physiological, pharmacological, and structural evidence that axons of hippocampal principal cells are electrically coupled, with prepotentials or spikelets forming the physiological substrate of electrical coupling as observed in cell somata. Antidromic activation of neighboring axons induced somatic spikelet potentials in neurons of CA3, CA1, and dentate gyrus areas of rat hippocampal slices. Somatic invasion by these spikelets was dependent on the activation of fast Na(+) channels in the postjunctional neuron. Antidromically elicited spikelets were suppressed by gap junction blockers and low intracellular pH. Paired axo-somatic and somato-dendritic recordings revealed that the coupling potentials appeared in the axon before invading the soma and the dendrite. Using confocal laser scanning microscopy we found that putative axons of principal cells were dye coupled. Our data thus suggest that hippocampal neurons are coupled by axo-axonal junctions, providing a novel mechanism for very fast electrical communication.