The mean pepsinogen I (PG I) level in a Finnish family sample was different in males and females and the difference was statistically significant. After exclusion of subjects with gastritis there remained 67 females and 68 males with morphologically completely normal antral and corpus mucosa. In females there was a significant increase of PG I with advancing age, the regression coefficient being 0.37 and statistically significant (p less than 0.01). In males no such increase was found, and individual cases revealed an almost random distribution with age. A similar increase with age has been noted in gastric acid output in females but not in males. Assuming that there is a linear relationship between PG I levels and the total chief cell mass, the PG I level would be determined by three main variables: thickness of the glandular layer, density of chief cells, and area occupied by chief cells. Of these variables the thickness and chief cell density showed neither in females nor in males any statistically significant increase with age, leaving the area as the variable which would account for the increase of PG I.