The distribution of certain catecholamines and indoleamines in the ventral nerve cord and the body segments of the medical leech, Hirudo medicinalis, was studied with the fluorescence microscope technique of Falck and Hillarp, with microspectrofluorometry, and with chemical determinations of the amines. The six cells of the segmental ganglia previously shown to be chromaffin were found to contain an amine, most probably 5-hydroxytryptamine. In the two giant cells, the amine was found on the surface of coarse intracellular granules, lying mainly at the cell membrane, and at the nucleus. The two giant cells send their axons to the body muscles, which thus seem to have a 5-hydroxytryptaminergic innervation. The four smaller amine-containing cells of the segmental ganglia send their axons to the neuropil of the ganglion.