Low molecular weight GTP-binding proteins (LMWG proteins) form a family of proteins that shows homology with Ras, are 18-30 kDa in mass, and bind and hydrolyze GTP. They act as molecular switches, being active when binding GTP. Their activity is regulated by other proteins that influence the dissociation of GDP and the rate of GTP hydrolysis. Roles are emerging for these proteins in regulation of membrane fusion and cytoskeletal organization and growth. In the gastrointestinal tract, the best studied physiological processes that may be regulated by LMWG proteins are digestive enzyme and gastric acid secretion.