Abstract: Gastrin is a peptide hormone that regulates both acid secretion and growth of the gastric oxyntic mucosa. Recent studies suggest that gastrin, in both its amidated, and less processed forms (glycine-extended gastrin and progastrin) may also exert biological activity in other organs in the gastrointestinal tract. This article will review the studies performed to date addressing the physiological role of gastrin outside of the gastric mucosa, with particular emphasis on the information gleaned from gastrin-deficient mice. Most of these studies address the potential role for the less processed forms of gastrin in regulating the proliferation of the colonic mucosa and colon cancers. There is also some data to support a potential role for gastrin in the regulation of the pancreas and the kidney, although the effects of gastrin deficiency on the function of these organs in mice have not yet been rigorously studied.