The aim of this experience has been to evaluate the chief cell mass and serum pepsinogen I in gastric ulcer patients. Comparisons were also made with parietal cell mass and acid secretion. Chief cell mass and serum pepsinogen I are not only influenced by the localization of ulcer but, also, by the histological condition of fundic mucosa. In fact, the behaviour of serum pepsinogen I and chief cell mass in type I gastric ulcer is the same observed in case of fundic chronic gastritis without gastric ulcer. In case of gastric ulcer type I with superficial fundic gastritis it emerges normozymogenism with hyperpepsinogenemia++ I, with preatrophic fundic gastritis hypozymogenism with normopepsinogenemia, with atrophic fundic gastritis hypozymogenism with hypopepsinogenemia I. In type II and III gastric ulcer the chief cell mass and serum pepsinogen I behaviour as they do in duodenal ulcer with hyperpepsinogenemia although hypozymogenism is present.