The distribution of 5-HT-like immunoreactivity in paraformaldehyde-fixed sections of retina, optic nerve and the optic lobe of Octopus vulgaris was studied by both immunofluorescence and avidin-biotin complex (ABC) immunohistochemical methods utilizing polyclonal antibodies to 5-HT. Some immunoreactive serotonin-containing cells were demonstrated in the retinal plexus, optic nerve, the ciliary body and the lens-generating tissue by both methods. An analysis of dissected retina and optic nerve of Octopus vulgaris by high pressure liquid chromatography (HPLC) with an electrochemical detector (ECD) also showed the presence of 5-HT. In the optic lobe, three 5-HT-immunoreactive bands in the plexiform layer of the cortex were clearly immunostained, and in the medulla both the cell islands and the neuropil contained some cells immunostained by both fluorescein isothiocyanate (FITC) and ABC methods. This is the first report on the systemic immunocytochemical visualization of 5-HT-containing cells and/or fibers in the cephalopod visual system.