Gastrin is thought to be a trophic agent for the digestive tract and the pancreas. This concept was studied on rats subjected to various operations designed to create hyper- or hypo-gastrinemia. Elevated serum gastrin concentration resulted from vagal denervation, antrum exclusion and fundectomy. Reduced serum gastrin concentration resulted from antrectomy. Antrum exclusion but not vagotomy increased the weight and height of the oxyntic mucosa; antrectomy had the opposite effects. There were no trophic changes in the small and large bowel or pancreas, attributable to the serum gastrin concentration. In the bypassed duodenum (following antrum exclusion or antrectomy B II) the villus height was greatly lowered suggesting that in this location the passage of food acts as a trophic stimulant. Our results show that endogenous gastrin exerts a trophic effect on the oxyntic mucosa; there was no evidence for a similar effect on the extragastric gut and pancreas.