Many chemotherapeutic agents that are effective against parasitic helminths affect cellular regulatory sites that control motility, metabolism, chemotaxis, and egg formation. Serotonin receptors are present in several species of parasitic flatworms and appear to participate in the regulation of motility and carbohydrate metabolism. In Fasciola hepatica these receptors are coupled to an adenylate cyclase through a cellular component that requires guanosine triphosphate. Serotonin is the most potent indoleamine agonist, while lysergic acid diethylamide and its 2-bromo derivative are the most potent antagonists. These studies are revealing additional sites in trematodes that may be important for the development of new and more selective chemotherapeutic agents.