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The possible existence of intrasynaptic ephaptic feedback in the invertebrate CNS was studied. Intracellular recordings were made of excitatory postsynaptic potentials and currents arising on activation of the recently described monosynaptic connection between identified neurons in the snail CNS. In the presence of ephaptic feedback, tetanization of the postsynaptic neuron with hyperpolarizing impulses should activate presynaptic calcium channels, thus increasing the amplitude of excitatory postsynaptic potential, while sufficiently strong postsynaptic hyperpolarization applied during generation of the excitatory postsynaptic current should induce “supralinear” increases in its amplitude, as has been observed previously in rat hippocampal neurons. The first series of experiments involved delivery of 10 trains of hyperpolarizing postsynaptic impulses (40-50 mV, duration 0.5 sec, frequency 1 Hz, train duration 45 sec); significant changes in the amplitude of excitatory postsynaptic were not seen. In the second series of experiments, changes in the amplitude of the excitatory postsynaptic current were studied during hyperpolarization of the postsynaptic neuron. At a potential of −100 mV, the amplitude of the excitatory postsynaptic current increased significantly more than predicted by its “classical” linear relationship with membrane potential. This “supralinear” increase in the amplitude of the excitatory postsynaptic potential can be explained by the operation of ephaptic feedback and is the first evidence for this phenomenon in CNS synapses of invertebrates.