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Each segmental ganglion of the leech nervous system has two pairs of lateral roots extending to the body wall and viscera. A cluster of about eight neuron cell bodies is located proximal to the first major branch of each anterior root and is termed the anterior root ganglion (ARG). Only one of these eight cells is vitally-stained by Neutral Red dye and fluoresces an intense blue-green following the glyoxylic acid histochemical condensation. The emission spectrum of this anterior root cell (AR) is stable under continuous UV illumination and is bimodal, with peaks at about 480 and 515 nm. This spectrum is indistinguishable from that of millimolar solutions of dopamine (DA) in gelatin droplets following glyoxylic acid histochemistry. We utilized high performance liquid chromatography and an amperometric detector to measure DA within the AR neurosomata at 1.01 pmol/cell. The AR cells in this study had an average diameter of 23 micron and therefore, the minimum intrasomatal concentration of DA is 160 mM, an unusually high level for any neurotransmitter. We measured DA in anterior axons at 0.83 pmol, in segmental ganglia at 1.07 pmol, and in longitudinal connectives at 0.16 pmol. Control neurosomata (Retzius cells) and axonal tracts which lack blue-green fluorescence (posterior and distal anterior roots), had no detectable DA (less than 0.06 pmol/sample). These data establish that the catecholamine DA is responsible for the fluorescence of the AR cell.