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1. Following the Mann-Williamson operation the acidity of the total secretions entering the stomach is much higher than before operation. During the gastric secretory curve the acidity of the secretions continues to rise and does not show the normal terminal decrease. The non-acid secretions entering the stomach are either very small in amount or absent. These changes are due to the absence of duodenal regurgitation. 2. The emptying time of the stomach is markedly increased after operation due apparently to jejunal spasm. 3. The acidity of the gastriccontents shows little change after operation. 4. Evidence is presented which indicates that the acidity of thetotal secretions (gastric plus duodenal) entering the stomach is controlled primarily by duodenal regurgitation while the acidity of the gastriccontents is controlled primarily by acid inhibition.

The physiological control of gastric acidity depends upon the following factors: 1. Strength of the acid secreted. 2. Regurgitation of non-acid secretions from the duodenum. 3. Inhibition of the intragastric chemical phase of acid secretion due to the presence of acid in the stomach and of the intestinal phase of acid secretion due to the presence of acid in the intestine. 4. The intensity of the stimuli arising in the pars pylorica and the degree to which, in a given animal, they influence the intragastric chemical phase of acid secretion. 5. Vagal or cephalic influences. 6. Absorption of water from the gastric contents.