Gastrin plays a central role in the regulation of acid secretion. It is released by meals in quantities sufficient to explain meal-stimulated acid secretion. Gastrin stimulates acid secretion mainly by releasing histamine from the enterochromaffin-like (ECL) cell. Whether gastrin has any functional direct effect on the parietal cell remains to be shown. Gastrin stimulates not only the function but also the growth of the ECL cell, and long-term hypergastrinemia may lead to ECL cell carcinoids. The role of the ECL cell in human gastric carcinogenesis is controversial, but it seems wise to avoid long-term iatrogen hypergastrinemia especially in young persons. Interestingly, the oxyntic mucosal D cell, on which gastrin has a negative trophic effect, may play a role in gastric stump carcinoma, and thus hypogastrinemia may also dispose to gastric cancer.