Abstract The sections in this article are: Gastrin and Cholecystokinin ( CCK ) Classification of Gastrin‐ CCK Receptors Locus of Receptors Modulation of Gastrin‐ CCK Receptors Postreceptor Mechanisms Applied Pharmacology in the Gut Substance P and Tachykinins Receptors to Tachykinins Degradation of Tachykinins Actions of Tachykinins Vasoactive Intestinal Polypeptide ( VIP ) Receptors Antagonists Control of Motility Opiates Natural Opiates Receptor Types Degradation of Opiates Opiates and Guinea Pig Ileum Actions on Segments of Gastrointestinal Tract in other Species Opiates, Immune System, and Gastrointestinal Motility Rational Design of Drugs Other Peptides Motilin Somatostatin Neurotensin Gastrin‐Releasing Polypeptide/Bombesin Family Galanin