A neuropeptide that strongly inhibits the spontaneous contractions of the oesophagus in Lymnaea has been characterized as GAPRFVamide. Direct mass spectrometry of nervous tissues and immunocytochemical studies show that the peptide is synthesized by neurones in the buccal ganglia and transported to the oesophagus via the dorso-buccal nerve. In accordance with the function of the peptide, immunoreactive fibres are detected within the muscle layer of the oesophagus. Finally, mass spectrometry reveals the presence of a number of unidentified peptides in the nerves that innervate the oesophagus, which suggests that oesophageal activities may be modified by multiple peptides.