Pentagastrin-stimulated gastric acid output and the histology of the antral and body mucosa were examined in 72 computer-selected probands and their 365 relatives, altogether 437 cases. The frequencies of upper abdominal complaints, peptic ulcer and hiatal hernia, and blood group distribution were comparable with those of the population at large. Acid output, expressed as mmol/h, mmol/h/kg of total body weight (TBW), mmol/h/kg of lean body mass (LBM), and mmol/h/kg of fat-free body weight (FFB), correlated with the changes in the body mucosa but not with those in the antrum. Acid output was lower in females than in males when expressed as mmol/h or mmol/h TBW but not when expressed in terms of FFB, which better than LBM accounts for the variation in the gastric surface area and, in this manner, for that of the parietal cell mass. This suggests that the lower acid output in females is due to a smaller gastric surface area and correspondingly smaller parietal cell mass rather than to a lower reactivity of the cells to stimulation. Acid output decreased with increasing age in both sexes, and this was due to a concomitant increase in the incidence and severity of atrophic changes in the body mucosa. An age-related decrease in acid output was not seen in males with a normal body mucosa. In females with a normal body mucosa, however, output expressed in terms of FFB showed a significant increase with age. The reason for this increase is not clear. In males and females with superficial gastritis of the body mucosa acid output decreased with increasing age regardless of the formulation used.