The diffusion of the radioimmunoassay methods for the determination of the serum pepsinogen has offered the opportunity of a direct approach to the analysis of peptic secretion. The aim of our studies has been to evaluate the behaviour of pepsinogen I in gastroduodenal pathology and to point out a possible clinical utilization. Moreover, we have examined all the relationship with the chief cell mass, parietal cell mass and hydrochloric acid secretion.