At present at least seven different endocrine cell types have been identified in the stomach. According to their relative frequency and secretion products the antral gastrin producing G cell and somatostatin producing D cell and the fundic histamine producing ECL cell are the best characterized cell types. Total endocrine cell mass is controlled by various factors from inside and outside the stomach. Density of antral G and D cells depends on the presence and absence of food, on the antral pH and on additional humoral and/or neural factors. Gastrin and not gastric pH has been identified as the most important factor regulating the density of fundic ECL cells. Adaptation of gastric endocrine cells to gastric pH and to the presence, abundance or absence of humoral and neural regulators are well known phenomena though only partially understood. Antral G cells increase and antral D cells decrease during long-term achlorhydria which as a consequence leads to hypergastrinaemia. Examples are pernicious anaemia in man and drug-induced acid suppression under experimental conditions. Interestingly, achlorhydria-induced G cell hyperplasia never progresses to gastrinomas. Fundic ECL cell density increases markedly in the presence of long-lasting hypergastrinaemia independently of gastric pH. In contrast to G cells ECL cell hyperplasia may progress to rarely occurring ECLomas. However, this depends on additional conditioning factors as the presence of severe atrophic gastritis as in pernicious anaemia or a specific genetic trait present in patients with gastrinomas associated with the MEN I syndrome.