Net Na+ flux from mucosa to lumen, potential difference, and volume and plasma protein outputs were measured in vagally denervated, separated pouches of the dog's oxyntic or pyloric glandular mucosa when the pouches were irrigated with Na+-free solutions whose pH ranged from 1.5 to 12.2. The apparent permeability to Na+(P'Na) was calculated. P'Na is lowest when the mucosa is bathed with acid and increases 2- to 3-fold when the pH is raised to 10. In the range of pH 10.0--11.2 P'Na is greater by an order of magnitude, but volume output is small, and no plasma proteins are shed. When the pH is above 11.2 there is an abrupt increase in P'Na, and the mucosa sheds a large volume of fluid containing plasma proteins. The change effected by raising the pH to the range of 10.0--11.2 occurs within 10 sec, and it is reversible. The change effected by raising the pH above 11.2 also occurs within 10 sec, and it is partly reversible.