The relationship between surface epithelial alkaline secretion and pH at the mucosal cell surface was studied in the duodenum of anesthetized rats. Alkaline secretion was measured by direct titration in situ using perfused segments of duodenum just distal to the Brunner gland area and devoid of pancreatic and biliary HCO₃^post−. Mucosal surface pH was measured by advancing a pH-sensitive antimony microelectrode from the luminal solution to the mucosal cell surface during continuous recording of pH. Acidification of the luminal solution markedly stimulated epithelial alkaline secretion: a change of luminal pH from 7.60 to 5.00 by ~100%, and from pH 7.60 to 2.00 by ~600%. Maximal pH in the immediate vicinity of the (luminal) cell surface remained at or slightly above neutrality during exposure to both luminal acidities. Prostaglandins (E₂, 16,16-dimethyl E₂, and F_2α, 3–140 μM luminally) increased the rate of alkaline secretion, surface alkalinity, and thickness of the pH gradient. Acetazolamide (40–80 mg/kg, i.v.) was a much more potent inhibitor of prostaglandin or acid-stimulated secretion than of basal alkaline secretion and decreased surface pH in acid-exposed duodenum. Aspirin (30 mg/kg, i.v.) had no effect on basal alkaline secretion (titrated at luminal pH 7.60) but significantly inhibited secretion at luminal pH 2.00, resulting in a decrease of surface pH. These data suggest that endogenous prostaglandins may be involved in mediating the alkaline response to luminal acid. Furthermore, because it is quantitatively sufficient to maintain neutral pH at the mucosal cell surface at luminal acidities normally encountered within the duodenal bulb, epithelial alkaline secretion presumably has an important role in duodenal protection against acid.