To clarify the mechanism of initiation of the hydrogen ion backdiffusion, the effects of aspirin and taurocholate, two representative gastric mucosal barrier breakers, on the potential difference, secretory activity, energy metabolism, and the hydrogen ion permeability of guinea pig gastric mucosa was studied in vitro. 1) The ATP content and energy charge of the gastric mucosa showed a statistically significant reduction when the potential difference decreased to one-half of that before addition. 2) The mucosal acid secretion was reduced by addition of the barrier breaker. 3) However, the hydrogen ion backdiffusion, as measured by titrating the acid appearing in the serosal solution, became detectable when the potential difference decreased to one-fourth of that before addition. It has been concluded, therefore, that the primary action of gastric mucosal barrier breakers is to damage the energy metabolism of the mucosal cells, and that the hydrogen ion backdiffusion takes place as the result of cellular death caused by the impairment of energy metabolism.