The lateral eye of the horseshoe crab, Limulus polyphemus, was studied by fluorescence microscopy according to Falck and Hillarp and microspectrofluorometry for identifying neuronal monoamines. After the formaldehyde treatment, the eccentric cells and their axons have a yellowish, rapidly fading fluorescence, such as is seen with 5-hydroxytryptamine. The microspectrofluorometric analysis was compatible with the fluorescence being caused by an indole, which could not be definitely identified, however. The eccentric cells have the ability to accumulate indoleamines such as 5-hydroxytryptamine, 6-hydroxytryptamine and 5,6-dihydroxytryptamine. Their axons were best demonstrated after being loaded with 6-hydroxytryptamine. Characteristic varicose terminals were seen in the neuropil, often arranged in clusters. Other terminals, possibly originating from the eccentric cells, were also seen among the pigment cells in the basal part of the ommatidia.