Abstract. In extracts of human gastric biopsies gastrin has been estimated with an immunochemical method. In the same biopsies G‐cells have been localized with an immunohistological method using peroxidase‐labelled antibodies and the endocrine cells investigated electron‐microscopically. Gastrin and G‐cells could be found regularly in the antral mucosa and only in insignificant amounts or not at all in the fundic mucosa of six normal persons. With the same methods gastrin and G‐cells could be demonstrated in the antral mucosa of rats and guinea‐pigs. The gastrin content of the antral mucosa of six patients with pernicious anaemia and achlorhydria with elevated serum gastrin levels was more than 20 times higher than in the controls and the G‐cells were significantly more numerous. Besides hyperplasia of the G‐cells, increased secretory activity was found electron‐microscopically. The gastrin release from the G‐cells seems to take place mainly via intracellular dissolution of the granule content within the membranous sacs. Although the number of other endocrine cells was increased in pernicious anaemia the ultrastructural identity of the G‐cells could be established by comparison with the cells of a Zollinger‐Ellison tumour. This tumour contained gastrin and gave a positive immunohistological reaction for this hormone. Also, the fundic mucosa of patients with pernicious anaemia contained gastrin and G‐cells, but considerably less than the antral mucosa. Hyperplasia of G‐cells was found in six cases of acromegaly, four of which also had a significantly increased gastrin content of the antral mucosa. This finding suggests a trophic function of the hypophysis, especially growth hormone, on the G‐cells. Hyperplasia of the G‐cells in the antral mucosa of three patients with primary hyperparathyroidism and increased gastrin content in two of the three cases also suggest a trophic function of the serum calcium level on the G‐cells.