Background/Aims: Spasmolytic polypeptide (SP) is a trefoil peptide expressed in the digestive tract. This study aimed to determine the structure and distribution of SP expression in the rat gastrointestinal tract. Methods: The structure of rat SP was determined from the sequence of complementary DNAs isolated from antral RNA. SP gene expression was localized by Northern blotting and in situ hybridization in the adult and fetal rat digestive tract. Expression of the SP peptide was localized by immunocytochemistry and Western blot analysis. Results: SP messenger (m)RNA was found predominantly in the stomach with highest expression in the antrum. High levels of SP mRNA were expressed in the fetal stomach before gastrin and somatostatin expression. Surprisingly, SP mRNA and peptide did not colocalize in the gastric mucosa, SP mRNA being superficial to SP peptide immunoreactivity throughout the gastric mucosa. Abundant SP immunoreactivity was seen in the lumen of the gastric glands and the mucus layer adherent to the gastric mucosa, indicating luminal secretion. Conclusions:In the rat, SP is a peptide secreted predominantly from antral mucous cell. The high concentrations of SP in the adherent gastric mucus layer (approximately 10 μmol/L) suggest that SP functions as a structural peptide rather than a regulatory peptide.