Although structurally simple, the nervous systems of parasitic helminths are biochemically complex. As well as the classical transmitters, the nervous systems of helminths contain a range of regulatory peptides and so far some 29 different peptides have been described. The only group of compounds thought to be transmitters in free-living organisms which have not yet been shown to be transmitters in helminths are the purines. There is evidence in helminths, as in vertebrates, for the co-localization, in the same cell, of classical transmitters and peptidergic molecules and for the non-neuronal occurrence of some regulatory peptides. Co-localization would effectively increase the number of transmitter types, since compounds could be released in different combinations. How such a selective release might be achieved is unknown. Different regulatory peptides are localized in different parts of the helminth nervous system, but it is quite probable that more than one peptide can occur in the same cell. Whether the relative proportions of the different peptides change under different physiological or developmental conditions is not known. The classical transmitters similarly show a differential distribution. It is presumed that the regulatory peptides in helminths, as in other organisms, are synthesized by post-translational modification of a high molecular weight precursor protein and that the peptides are not recycled but are removed from their site of action by specific peptidases. The latter are located near the receptor sites and may be membrane-bound. In vertebrates the precursor molecules usually contain several different peptides with different biological functions and processing of the precursor often shows tissue-specific variations.