PHYSIOLOGISTS have long pondered the riddle of why the stomach is itself not digested by the very juice it secretes. One explanation is that a mucus-bicarbonate barrier, coating the stomach lumen as well as superficial portions of gastric glands, prevents autodigestion(1). However, this leaves unanswered the question of what protects cells deeper in the glands, which seem to lack a mucus barrier(2). These are the parietal and chief cells, which secrete acid and pepsin. Using perfused single gastric glands from rabbit, we recently found that intracellular pH is uniquely resistant to extreme degrees of luminal acidification(2), suggesting that the apical (luminal) barrier might also exclude ammonia and carbon dioxide, to which cell membranes are generally highly permeable(3,4). We now show that this is indeed the case. There are three reports of membranes with very low permeabilities to NH3 (refs 5-7), and none of membranes impermeable to CO2.