Bile salts break the gastric mucosal barrier. To explain this, the suggestion has been made that bile salts may disrupt surface epithelial cell membranes or break the tight junctions between cells, but appropriate ultrastructural studies are lacking. We therefore instilled control and bile salt-containing solutions into the stomachs of fasted mice at pH, 1, 3, 5, AND 7. Taurocholate (pKa equals 1.8) caused mucosal injury only at pH 1, whereas glycochenodeoxycholate (pKa equals 4.2) injured the mucosa at pH 1 and 3. By electron microscopy, areas of mild mucosal injury were characterized by clumping of nuclear chromatin and loss of cytoplasmic density within surface mucous cells. The apical cell membranes and tight junctions remained intact. In areas of severe damage surface cells were ruptured but tight junctions still appeared unbroken. These studies indicate that acid pH markedly augments the damaging effects of bile salts on mouse gastric mucosa. Moreover, as an initial step in the mechanism of bile salt-induced gastric injury, the nonionized moiety of a given bile salt which exists below its pKa may be important in altering the gastric surface epithelial cell in a way which allows the ingress of bile salt and/or hydrogen ion to cause intracellular damage.