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Abstract— We have measured serotonin (5‐HT) within large and small neurosomata which are vitally stained by Neutral Red dye. A micro‐radioenzymatic technique which is sensitive to 50fmol of 5‐HT was employed on intact ganglia, 75 μm Retzius Cells (RZ) and a 10 μm ventro‐lateral cell (VL) taken from the leech Macrobdella decora. The stain does not affect the levels of 5‐HT in either ganglia or RZ. The VL cell body contains 5‐HT at concentrations of at least 100 mm. Microspectrofluorometry of all the ganglionic neurosomata which fluoresce following the Falck‐Hillarp formaldehyde condensation reaction detected rapidly‐fading emission peaks of 509–523 nanometers. We conclude that all seven fluorescent neurons in the leech ganglion very probably contain serotonin.