There is evidence that under various physiological circumstances long-term adaptation of structure and function of the gut occurs. The mechanisms of these changes are not clear but gastro-intestinal hormones may be involved. In particular, gastrin which has been shown experimentally to stimulate growth and development of parts of the gut, probably has a role in maintaining the structure of the normal upper alimentary tract. Cholecystokinin may be of major importance in producing adaptive changes in the pancreas in response to dietary modifications and enteroglucagon is possibly concerned with maintaining a normal small intestinal structure. The importance of the 'trophic' action of gatrointestinal hormones is becoming more widely recognised and as new gastro-intestinal hormones become established, this aspect of their physiological importance as well as their acute effects, will deserve attention.