In in vitro studies on piglet gastric mucosa, the replacement of Na in the bathing solution on the mucosal side by choline caused a decrease in electrical potential difference (p.d.) and an increase in mucosal resistance, implying Na transport from mucosa to serosa. Na transport was reduced during acid secretion, but the rate of secretion was unaffected when Na movement was inhibited by the addition of the diuretic amilaride. Replacement of Cl with isethionate on the mucosal side caused an increase in p.d. and resistance. Unidirectional fluxes of C1 were measured with 36Cl separately or in double isotope experiments with 24Na. The movement of Cl flux from serosa to mucosa was dependent on the concentration of Cl in the respective solutions, and the rate of movement increased during maximal stimulation of acid secretion with histamine, and decreased when secretion was inhibited by anaerobiosis. At rest, on open circuit, Na and Cl moved from mucosa to serosa, and a low level of H secretion occurred.