Gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the pyloric antrum of the stomach, duodenum, and the pancreas.

Gastrin binds to cholecystokinin B receptors to stimulate the release of histamines in enterochromaffin-like cells, and it induces the insertion of K⁺/H⁺ ATPase pumps into the apical membrane of parietal cells (which in turn increases H⁺ release into the stomach cavity). Its release is stimulated by peptides in the lumen of the stomach.

In humans, the GAS gene is located on the long arm of the seventeenth chromosome (17q21).

Gastrin is a linear peptide hormone produced by G cells of the duodenum and in the pyloric antrum of the stomach. It is secreted into the bloodstream. The encoded polypeptide is preprogastrin, which is cleaved by enzymes in posttranslational modification to produce progastrin (an intermediate, inactive precursor) and then gastrin in various forms, primarily the following three:

Also, pentagastrin is an artificially synthesized, five amino acid sequence identical to the last five amino acid sequence at the C-terminus end of gastrin. The numbers refer to the amino acid count.

Gastrin is released in response to certain stimuli. These include:

Gastrin release is inhibited by:

The presence of gastrin stimulates parietal cells of the stomach to secrete hydrochloric acid (HCl)/gastric acid. This is done both directly on the parietal cell and indirectly via binding onto CCK2/gastrin receptors on ECL cells in the stomach, which respond by releasing histamine, which in turn acts in a paracrine manner on parietal cells stimulating them to secrete H+ ions. This is the major stimulus for acid secretion by parietal cells.