In males with a normal gastric mucosa the acid secretion expressed in terms of fat-free body weight (FFB) shows a random distribution with age, whereas in females there is a significant increase. To explain this sex-related discrepancy, we examined maximal pentagastrin-stimulated gastric acid output (AO) and the parietal cell density (P) and thickness (T) in 50 males and 50 females with a normal antral and body mucosa randomly collected from a representative Finnish population sample. Two alternatives were considered: 1) the parietal cell reactivity remains constant, and the age variation in females is due to an increase in the total parietal cell mass, 2) the age variation in females is due to an increase in the reactivity of parietal cells, the total parietal cell mass (P X T X body area (A)) remaining constant. Starting from the first alternative, P and T were measured and showed no correlation with age in males and a decrease in females. The variable A is, according to the formula, expressed by the ratio AO/P X T, and this ratio shows an expected significant correlation with AO in both males and females. However, the variable A showed a significant increase with age in females but not in males. In view of the results of Cox that A should not vary with age, the present results suggest that the ratio AO/P X T in females must include, in addition to body area, other variables, such as the reactivity of parietal cells.(ABSTRACT TRUNCATED AT 250 WORDS)