Experimental work on the effect on gastric acidity of section of the vagus nerves has been largely inconclusive for 3 reasons: (1) the preoperative normal state has not been adequately determined; (2) the observations often were made on animals that had been operated on for the production of fistulas and pouches, and (3) the period of postoperative observations has been too brief. The present study, which was carried out on 8 dogs, is concerned only with the effect of section of the vagus nerves on the degree of acidity of the gastric content during that portion of the normal digestive cycle which begins with the ingestion of a meal and which ends when the stomach is empty. It was found necessary to train each animal for 1-6 wks. in order to have it thoroughly accustomed to the procedure. In the beginning, any psychic upset, even intubation, was sufficient to cause a marked decrease in acidity. Only at the end of the period of training did the acidity begin to show a uniform curve. Section of the vagus nerves then was performed. In some experiments the large anterior and posterior vagal trunks were cut just above the diaphragm; in others, the vagal branches were cut in the abdomen, care being taken to cut the pyloroduodenal nerve which descends from the vagal branch to the liver. In those dogs in which the vagus nerves were sectioned in the thorax, there was marked reduction in both free and total acidity, and the highest postoperative curve was lower than the lowest preoperative curve. There was 1 exception: in 1 animal the concentration after operation was the same as before, and at necropsy it was found that the posterior vagal trunk had been missed. In those animals in which the branches of the vagus nerves were cut in the abdomen the amount of free HCl occasionally continued to rise to the same height as before operation. However, the secretory curve was shorter, so that the stomach was empty of all food and gastric juice 1-2 hrs. sooner than before operation.