The constituent elements of the gills of Aplysia kurodai and A. juliana were examined for the presence of biogenic amines using histochemical, immunocytochemical, and HPLC techniques. Aminergic elements were revealed by glyoxylic acid-induced fluorescence in the branchial nerve, branchial ganglion, branchial vessels, and pinnules in both species. Three types of fluorescent cells were found in the neural plexus of the gill in each species. Two of them might be sensory neurons. Although HPLC analysis showed the presence of serotonin and dopamine in all gill structures including fluorescent neural elements, there were regional differences in concentrations of the monoamines. It was noted in the pinnules that there was a much higher concentration of dopamine than serotonin. Serotonin immunocytochemistry revealed neural processes which were immunoreactive to antiserotonin antibody, but serotonin immunoreactivity could not be found in a population of branchioganglionic neuron (BGN) somata. Serotonergic elements in the ganglion may be processes of the central ganglion, while dopaminergic elements may be processes of neurons in the neural plexus, located beyond the branchial ganglion. BGNs were activated by bath-applied dopamine and serotonin. These results suggest that dopaminergic sensory inputs from the neural plexus and serotonergic descending inputs from the abdominal ganglion may be among the inputs received by BGNs. It was found that serotonin depressed excitatory junctional potentials in muscle cells of the efferent branchial vessel, which were induced by an identified neuron of the abdominal ganglion. The aminergic cellular organization of the gill may involve serotonergic presynaptic-inhibitory fibers arising from the abdominal ganglion.