In the present study, a counting method for the evaluation of chief cell mass was proposed, based on the assessment of a zymogenous index (ZI) obtained by multiplying the number of cells/mm2 by the thickness of the glandular layer. Results obtained in 40 subjects with normal gastric mucosa did not show significant ZI differences between sexes. A statistically significant decrease in ZI was observed in patients above age 50, thus being directly related to the significant decrease in the thickness of the glandular parenchyma and in the number of chief cells/mm2 observed in this age group. The data obtained were in agreement with the pattern of change observed for serum pepsinogen I (PG I) whose values were related to the chief cell mass in connection with sex and age of the subjects.