Gastric mucosa may protect itself from acid peptic digestion by maintaining an alkaline barrier zone within the layer of mucus coating its surface. We have measured the pH gradient in the mucous layer in vivo, on the gastric mucosa of anaesthetised rats using antimony chloride micro pH electrodes. The maximum pH recordable adjacent to the epithelium was 7.43 +/- 0.56 (n = 8) when the luminal bathing solution pH was 2. Adjusting the luminal pH to 7.0 caused the maximal pH to rise to 7.88 (range 7.59 to 8.08), a value which is significantly higher than either luminal or reported intraepithelial pH and suggests that active secretion of alkali is involved. Pretreatment with 16-16-dimethyl prostaglandin E2 (20 micrograms subcutaneously) significantly increased the maximal intramucus pH to 7.89 +/- 0.45 (n = 8) when luminal pH was 2 and prevented the fall in intramucus pH induced by luminal aspirin (20 mM). It did not prevent falls in pH provoked by the mucolytic agent n-acetyl cysteine or by a high luminal activity (pH 1.4). These data indicate that an alkaline environment is maintained adjacent to gastric mucosa and that while this is enhanced by prostaglandin it may be compromised by high luminal acid concentrations or by removal of the support provided by mucus. These observations may be relevant to the mechanisms of gastric mucosal protection against acid peptic damage.