The isolated gastric mucosa of the frog will secrete acid when mounted in a bath consisting of 2 chambers separated by the gastric membrane. Under these conditions the glandular tissue of the frog''s stomach resembles smooth muscle tissue in its sensitivity to alteration in its ionic environment. The descending order of sensitivity to the various ions is as follows: Ca, K, Mg, PO4, and HC03. These results reveal that an external supply of PO4 and HC03 ions is not necessary for the formation of acid by the frog''s stomach.