Auditory afferents terminating as mixed, electrical, and chemical, synapses on the goldfish Mauthner cells constitute an ideal experimental model to study the properties of gap junctions in the nervous system as well as to explore possible functional interactions with the other major form of interneuronal communication--chemically mediated synapses. By combining confocal microscopy and freeze-fracture replica immunogold labeling (FRIL), we found that gap junctions at these synapses contain connexin35 (Cx35), the fish ortholog of the neuron-specific human and mouse connexin36 (Cx36). Conductance of gap junction channels at these endings is known to be dynamically modulated by the activity of their co-localized chemically mediated glutamatergic synapses. By using simultaneous pre- and postsynaptic recordings at these single terminals, we demonstrate that such functional interaction takes place in the same ending, within a few micrometers. Accordingly, we also found evidence by confocal and FRIL double-immunogold labeling that the NR1 subunit of the NMDA glutamate receptor, proposed to be a key regulatory element, is present at postsynaptic densities closely associated with gap junction plaques containing Cx35. Given the widespread distribution of Cx35- and Cx36-mediated electrical synapses and glutamatergic synapses, our data suggest that the local functional interactions observed at these identifiable junctions may also apply to other electrical synapses, including those in mammalian brain.