Distribution of serotonin (5-HT) content of nervous fibers in both the somatic and the visceral muscle of Eisenia fetida have been investigated using immunocytochemical staining and voltammetric measurements. The somatic muscles in the body wall are richer innervated with serotoninergic fibers than the visceral ones in the pharynx and gizzard. The relative density of immunopositive fibers in the circular muscle layer of the body wall was found to be 2.73% while in the prostomium it was 1.02%. In the case of the muscle in pharynx 1.12% and in gizzard 1.28% density values were found. Differential Pulse Voltammetric (DPV) measurements with carbon fiber electrodes in the above mentioned muscle layers gave 272.5 nA, 135.0 nA, 122.5 nA, 137.5 nA peak heights, respectively. In the statistical analysis T-test was used at a confidence level of 95% (p<0.05). DPV current peak (i(p)) values reflect clearly the 5-HT concentration differences. Significant correlation was found between the innervation density and the i(p) values recorded in different areas. The i(p) values recorded at different times in different locations are determined by instantaneous serotonin concentration of the living tissue. As far as we know this is the first report using in vivo voltammetry investigating serotonin content in earthworm, E. fetida.