Cholecystokinins are a series of peptides of heterogeneous length (5 to 58 amino acids) that are derived from preprocholecystokinin and are found in gastrointestinal tissues and the central nervous system. Gastrin is a related peptide with 5 C-terminal amino acids identical to those of cholecystokinin. Two GPCRs, CCK1 (CCKA) and CCK2 (CCKB), bind to CCK and/or gastrin to mediate the biological effects of the peptides. CCK1 selectively binds sulfated CCK, whereas CCK2 binds to CCK and gastrin with similar affinity. Binding of ligands to CCK1 stimulates mobilization of intracellular calcium by activation of Gq/11. CCK1 receptors in the periphery are primarily localized in the pancreas, gallbladder, pylorus, and intestine where they are responsible of the regulation of diverse digestive processes. They are also present in select areas of the peripheral nervous system (vagus nerve), and the CNS where they mediate the satiety effects of CCK, regulate an increase in dopamine release, and antagonize opiod analgesia.