The presence and localization of the serotoninergic and FMRFamidergic structures in the nervous system of the trematode Opisthioglyphe ranae, the marsh frog intestinal parasite, was studied using immunocytochemistry. The serotonin-immunoreactive nerve cells and fibers were revealed in the head ganglia, circular commissure, longitudinal nerve cords and their connective commissures, as well as around the oral and ventral suckers, esophagus and genital pore. FMRFamide immunoreactivity was observed in the head ganglia, longitudinal nerve cords and terminal parts of the reproductive system. The results obtained are discussed in the light of the available data on the presence and functional significance of the above neurotransmitters in trematodes. signaling substances in trematodes of different taxa, in the present work the presence and distribution patterns of serotoninergic and FMRFamidergic components was examined in the nervous system of the trematode Opisthioglyphe ranae inhabiting the frog intestine. MATERIALS AND METHODSThe trematode Opisthioglyphe ranae (Fr lich, 1791; Plagiorchiidae) is a widespread intestinal parasite of amphibians. It has a trixenic life cycle.