In Lymnaea stagnalis integumental Na+ uptake is stimulated by the sodium influx stimulating (SIS)-peptide. Its primary structure was determined as: SRTQSRFAS- YELMGTEGTECVTTKTISQICYQCATRHEDSFVQVYQECCKKEMGLREYCEEIYTELPIRSGLWQPN++ +. Antisera raised against parts of SIS-peptide stained neurons in the visceral, parietal, and pleural ganglia, and in the proximal parts of the intestinal, anal, and right internal pallial nerves. Locations and axon projection patterns of these neurons suggest that they represent the previously described neurosecretory yellow cells.