Studies were conducted with in vivo and in vitro canine stomach preparations. Instillation of 5, 10, and 20 mM bile salts in TES bufer (pH 7.4) into the nonsecreting stomach in vivo caused a progressive decrease in electrical potential difference (PD) and an increase in electrical resitance (R). The rate of acid secretion, determined by the pH-stat method in the histamine-stimulated stomach, decreased with 5 and 20 mM bile salts. Mucosal adenosine triphosphate (ATP) content of the nonsecreting or secreting stomach was reduced by bile salts. In vitro flux studies demonstrated that within the first hour after 1 mM bile salts were added to the mucosal side of the chamber, PD decreased, R increased, and net sodium transport decreased. In the second hour, unidirectional fluxes of sodium increased, indicating an increase in permeability of the gastric mucosa to sodium. These results demonstrate that the initial action of bile salts is inhibition of ion transport, which is followed by an increase in permeability.