Studies were made in seven dogs of the gastric mucosal barrier before and after making a gastric ulcer by surgical excision of gastric mucosa. In four anesthetized and atropinized dogs (acute studies) with the distal esophagus and pylorus ligated, an acid-saline mixture (100 mM HCl + 54 mM NaCl) was instilled into the stomach in consecutive 20-min periods before and after excising gastric mucosa to make an ulcer. Net movements of H+, Na+, K+ and Cl- ions were not significantly altered following the ulcer. In further studies in three dogs (chronic studies), transmucosal potential difference, net movements of electrolytes and maximal acid output were also unchanged by making a gastric ulcer.