Certain intact nerve cells in flies can be filled with cobalt from presynaptic or postsynaptic neurons. This cobalt coupling is best demonstrated in giant fibre systems where the phenomenon was originally termed 'transsynaptic staining'. Fine structural analysis of silver-intensified, cobalt-coupled neurons indicates that the passage of cobalt ions occurs at gap junctions that are accompanied by conventional chemical synapses. Cobalt-coupled systems in dipterous insects are uniquely identifiable and can always be detected between the same kinds of neurons. The visualization of cobalt-coupled neurons allows the identification of functional pathways linking the brain to motor neuropils.