Primary afferent fibers from the electroreceptors of mormyrid electric fish use a latency code to signal the intensity of electrical current evoked by the fish's own electric organ discharge (EOD). The afferent fibers terminate centrally in the deep and superficial granular layers of the electrosensory lobe with morphologically mixed chemical-electrical synapses. The granular cells in these layers appear to decode afferent latency through an interaction between primary afferent input and a corollary discharge input associated with the EOD motor command. We investigated the physiology of deep and superficial granular cells in a slice preparation with whole cell patch recording and electrical stimulation of afferent fibers. Afferent stimulation evoked large all-or-none electrical EPSPs and large all or none GABAergic IPSPs in both superficial and deep granular cells. The amplitudes of the electrical EPSPs depended on postsynaptic membrane potential, with maximum amplitudes at membrane potentials between -65 and -110 mV. Hyperpolarization beyond this level resulted in either the abrupt disappearance of EPSPs, a step-like reduction to a smaller EPSP, or a graded reduction in EPSP amplitude. Depolarization to membrane potentials lower than that yielding a maximum caused a linear decrease in EPSP amplitude, with EPSP amplitude reaching zero mV at potentials between -55 and -40 mV. We suggest that the dependence of EPSP size on postsynaptic membrane potential is due to close linkage of pre- and postsynaptic membrane potentials via a high conductance gap junction. We also suggest that this dependence may result in functionally important non-linear interactions between synaptic inputs.