Neuropeptide myodulin, small cardiac active peptide (SCP) and buccal protein are widely distributed in the phylum mollusca and have important physiological functions. Here we describe the detailed distribution of each peptide class in the central nervous system (CNS) of the Lymnaea stagnalis by using immunocytochemical techniques in combination with dye labeling of neurons identified by electrophysiology. We report on the isolation and structural characterization of A Lymnaomodulin PMSMLRLamide, which is identical with myomodulin A from the sea hare. Myomodulin immunoreactivity is located in all 11 ganglia, their connective tissue, and peripheral nerves. In many cases, myomodulin immunoreactivity appears in clusters of neurons expressing FMRFamide-like peptides, but also in a large number of other neurons. Double labeling assay demonstrated the immunoreactivity of myomodulin in visceral white interneurons involved in cardiopulmonary regulation. Scp-like immune reactivity is also present in all ganglia, and double labeling assays show that it is associated with cluster specificity of different exons expressed differentially in the CNS of the FMRFamide gene at many locations. However, the characteristics of the two SCP antiserums used in this study suggest that they cross-react with FMRFamide and n-terminal extended FmrFamide-like peptides. Selective preadsorption using these cross-reactive peptides resulted in the elimination of extensive staining and the retention of true SCP immune reactivity only in buccal and foot ganglia. Buccal protein immunoreactivity was limited to buccal and foot ganglia. It is not consistent with the distribution of myomodulin or SCP. Most immune response clusters are found in the foot ganglion. It is not consistent with the distribution of myomodulin or SCP. Most immune response clusters are found in the foot ganglion. It is not consistent with the distribution of myomodulin or SCP. Most immune response clusters are found in the foot ganglion.