Backdiffusion of H+ was studied in isolated bullfrog gastric mucosa using direct titration to evaluate changes in luminal H+ level. In resting mucosae, the resulting permeability coefficient for H+ (uncorrected for electrical effects) was 0.4 X 10(-5) cm X S-1, which is of the same order of magnitude as that estimated from data in the literature for dog and rabbit gastric mucosa in vivo. With maximal stimulation of acid secretion, backdiffusion of H+ was considerably increased. In both resting and stimulated mucosae, an increase in Cl- flux from lumen to serosa was observed to accompany H+ backdiffusion. In experiments in which electrical activity of resting mucosae was monitored, instilled H+ lowered gastric potential difference but had little effect on the normal resistance. Sudden drops in resistance and potential were observed, independent of luminal acid; after such events instilled H+ had no effect on mucosal resistance. It appears that a substantial part of H+ backdiffusion occurs as the ion pair HCl.