Detailed comparative-histological data on localization and morphological peculiarities of catecholamine-, serotonin-, neurotensin- and FMRFamide-containing elements in the nemertine digestive system are presented for the first time. Using fluorescent histochemistry and immunohistochemistry, pharynx, esophagus, and midgut were studied in five species from three genera of the White Sea nemertines. Described in various nemertine species are intra- and subepithelial cells of the open and closed type, containing biologically active substances. Their processes are distributed in basi- and subepithelial nerve plexuses and can make contact with lumen of the digestive tract. Species-specificity in localization of the cells containing certain biologically active substances is noticed both along the length of the tract and with respect to its epithelial layer. Peculiarities and common regularities in distribution are discussed, as well as a possible function of monoamine- and peptide-containing elements in the digestive tract of the studied nemertines and other invertebrates.