Extracts were prepared from selected neurons taken from the central ganglia of the pulmonate snail Helix aspersa, and parallel radioimmunoassay (RIA) and high pressure liquid chromatography (HPLC) were used to analyze for FMRFamide-related peptides (FaRPs). Some neurons (e.g., the cerebral C3 neuron) contained only the tetrapeptides related to FMRFamide (tetra-FaRPs), whereas two clusters of neurons in the parietal ganglia contain a preponderance of the hepta-FaRPs. Using cloned cDNA probes in molecular hybridization studies, we showed that the C3 neuron (and some other large neurons) contain the mRNA species that encodes the tetra-FaRPs. In contrast, the neurons of the parietal clusters predominantly express the mRNA that encodes the hepta-FaRPs. These results were confirmed by in situ hybridization with antisense RNA on whole mount ganglia. Each type of experiment supports the view that FaRP-containing Helix neurons express either the tetra-FaRPs or the hepta-FaRPs, but not both.