The authors, after having set out the classification and general significance of endogenous opioids (e.o.) on the basis of the most recent researches, have focused attention on the activity of the opioid peptides in gastroenterology. They summarize the main effects of the opioids on the basic digestive functions: secretion, endoluminal digestion, motility, absorption. Particular importance is attributed to the activity of the e.o. on gastric and duodenal secretion. Finally, the possible therapeutic implications of e.o. in the management of digestive disorders are examined.