AbstractNaCl solutions were instilled into ligated unstimulated whole stomach pouches in cats for 30 min. On instillation of distilled water or 310 or 620 mM NaCl the net fluxes of NaCl were roughly proportional to the mean concentration difference between the gastric lumen and the blood. On instillation of more than 620 mM NaCl the absorption was larger than expected from this proportionality, a result believed to be due to an increase in the permeability of the gastric mucosa on instillation of such hyperosmotic solutions. Instillation of distilled water or 310 or 620 mM NaCl had only minor effects on the net flux of hydrogen ions out of the stomach during subsequent instillations of 170 mM HCl, In comparison with the controls. On instillation of HCI subsequent to an instillation of 620 mM NaCl the net fluxes of sodium ions and water into the stomach were significantly increased over the control level and there was a net influx of chloride ions instead of a net efflux. This result was probably due to secretion of a NaCl containing fluid. On instillation of 170 mRI HCl subsequent to an instillation of 930 or 1550 mM NaCl the net effluxes of hydrogen ions also were significantly increased over the control levels. The concentration of NaCl in the instilled solution needed to obtain an effect on the gastric mucosal ion permeability, both during the instillation of NaCl and on subsequent instillation of 170 mM HCl, was thus larger than 620 mM.