The consequences of exposure of the intact stomach to intestinal contents were examined in six dogs. Diversion of duodenal contents through the stomach lead to the following changes: histologic gastritis in both antrum and corpus, increase in resting and postprandial serum gastrin levels, increased parietal cell density in four of six animals, and enhanced maximal acid secretory capacity in three of six animals. No significant changes were seen in insulin-stimulated acid secretion, insulin-stimulated pepsin secretion, antral gastrin levels, or G cell numbers. We conclude that chronic exposure of the intact stomach to duodenal contents results in gastritis and an amplified gastrin response to food. Parietal cell numbers and maximal acid secretory capacity are increased in some animals.