The monogenean flatworm Polystoma nearcticum exhibits reproductive synchrony with its treefrog host, Hyla versicolor, and becomes reproductively active only during the short period of host sexual activity at spawning. In this way, it provides a useful model system for exploring factors that may influence egg production in flatworm parasites. One such factor is the peptidergic innervation of the egg chamber or ootype. By using immunocytochemical techniques, the occurrence and distribution of GYIRFamide-like immunoreactivity, an authentic flatworm FMRFamide-related peptide (FaRP), have been monitored in the cells and fibres innervating the reproductive apparatus of worms collected at different stages of host sexual activity. Serotonin (5-HT) immunoreactivity in the worm was mapped for comparison. Extensive immunostaining for the FaRP and 5-HT was obtained throughout both the central and the peripheral nervous systems of worms, which were recovered from reproductively active frogs. In contrast, the innervation of the ootype of worms that were determined to be sexually inactive, including those recovered from frogs postspawning, showed little or no immunoreactivity for the FaRP; immunostaining for 5-HT in the ootype was unaffected by the reproductive state of the worm. These results indicate that FaRP expression in the neurons of the ootype innervation of P. nearcticum coincides with the parasite's brief period of egg production and, thus, provides evidence that regulatory peptides may be involved in the egg-assembly mechanism in flatworm parasites.