Chief cell mass and serum Pepsinogen I were calculated in 25 duodenal ulcer patients and in 40 healthy controls. Comparisons were also made with parietal cell mass and the stimulated acid secretion. Chief cell mass was expressed bay a Zymogenous Index (ZI) obtained by multiplying the number of chief cells per mm2 for the thickness of the glandular layer. The analysis of the data shows a significant decrease of ZI in duodenal ulcer patients in comparison with controls. The grouping of the subjects according to the age (over and under fifty years), confirms that ZI decreases significantly in duodenal ulcer groups in relation with the respective groups of control. In case of duodenal ulcer with fundic superficial gastritis no behavioural changes of ZI are observed in comparison to those affected by duodenal ulcer with normal fundic mucosa. The serum Pepsinogen I significantly increases in duodenal ulcer patients in comparison to the controls, either in the subjects over and under fifty. In the group of duodenal ulcer patients with superficial fundic gastritis a significant increase of serum Pepsinogen I can be pointed out in comparison to the ulcer patients with normal fundic mucosa. The grouping of the patients on the basis of hyperparietalism and normoparietalism has shown in the former group hypozymogenism and in the latter normozymogenism. Finally, no correlation has emerged between Zymogenous Index and serum Pepsinogen I.