The gastric gel mucus layer plays an important role in the defense against acid-induced injury. Previous investigations have modeled the interaction between the mucus, bicarbonate, acid moving towards the epithelial surface, and water carrying ions away from the epithelium. We measured the diffusion coefficient of protons through gastric mucus and applied this parameter to the model. Using the measured diffusion coefficient, we applied the model to previously published gastric mucus gel pH gradients to assess the relative contribution of bicarbonate or water fluxes in maintaining the gradient in the face of luminal acid. Mucus was harvested from the stomachs of anesthetized rats, mixed with bromphenol blue, and placed in capillary tubes. The tubes were submerged in 0.15 N HCl at 37 degrees C. The color change from blue to yellow was measured with time as the HCl migrated through the tube. From the distance and time data, the diffusion coefficient could be calculated. Controls of bromphenol blue in water were studied. The diffusion coefficient for water was 30.2 +/- 0.8 x 10(-6) cm2 s-1, almost identical to the standard value reported in the literature. The HCl diffusion coefficient through saline was 67.8 +/- 0.7 x 10(-6) cm2 s-1. For gastric mucus it was 5.4 +/- 1.3 x 10(-6) cm2 s-1. Application of this diffusion coefficient to the model and subsequently to observed pH gradients suggested that the most potent defense mechanism against concentrations of luminal acid is the bulk movement of water away from the epithelial surface through the mucus gel layer.