This paper provides an overview of research on the nervous system of parasitic platyhelminths. We have emphasized studies concerned with the physiological, pharmacological and biochemical nature of the major small molecule neurotransmitters of these parasites. We have attempted to provide a critical review of the work by focusing on important unresolved issues. Finally, we have focused on some recent work in our laboratory, using patch-clamp recording techniques and quantitative fluorescence cytometry, as an example of newer methods that will hopefully resolve some of the unanswered questions concerning the nervous system of these parasites.