In the present study co-localization of serotonin and FMRFamide-like immunoreactivities was studied in the central nervous system of the earthworm, Eisenia fetida. Alternate serial sections were stained with antisera raised against serotonin and FMRFamide, using the peroxidase-antiperoxidase method. The immunostained consecutive sections were compared by light-microscopy and on reconstructed images using NIH Image and Neurolucida programs. Although there was a large number of serotonin and FMRFamide-like immunoreactive neurons in the central nervous system of Eisenia, co-localization of immunostaining was relatively rare. In the cerebral ganglion no cells were found that stained for both antisera. In the subesophageal and ventral cord ganglia the neurons reacting for both antisera represented approximately 17-23% and 10-14% of the serotonin- and 7-11% and 9-17% of the FMRF-amide-like immunoreactive cells, respectively. It is suggested, that FMRFamide-related peptides act as neuromodulators and may also influence the activity of serotonergic neurons.