In general, most flatworms contain very little histamine (HA) and their nervous systems often lack, or contain very few, histaminergic elements. However, preliminary studies in our laboratory have revealed that the frog lung parasite, Haplometra cylindracea (Trematoda: Digenea), contains HA in a very high concentration. For this reason, the present study was undertaken to study the localization and synthesis of HA in this worm by using immunocytochemistry and high-pressure liquid chromatography (HPLC). Essentially all parts of the nervous system of H. cylindracea showed HA-like immunoreactivity. The paired cerebral ganglia and nerves emanating from these, including the longitudinal nerve cords, were intensely immunoreactive. The musculature of the pharynx, oral and ventral suckers, and those of the reproductive organs were all innervated by HA-immunoreactive fibers. Fiber plexuses beneath the tegument and throughout the parenchyma also showed HA-like immunoreactivity. HPLC studies revealed one of the highest HA concentrations in the animal kingdom, 6.49 t 1.36 nmoleimg protein, in the worm. The frog lung and blood contained very low concentrations of HA and could be excluded as sources for HA, while an enzyme assay revealed that the worm produces HA by decarboxylation of histidine. Thus, it is likely that H. cylindracea uses HA as a neurotransmitter or modulator.