1. The inner lining of the gizzard was degraded by in vitro pepsin treatment at pH values lower than 2. 2. A significant linear regression was observed between the pH values on the surface of the inner lining and those on the gizzard mucous layer just under the lining. 3. The control chicks had a specific distribution pattern of pH values on the lining of the gizzard. By feeding a gizzard erosion inducer, pH values were lowered and the distribution pattern disappeared. In the latter case, however, a gastric erosion was observed consistently at the area which showed the lowest pH value in the control chicks. 4. From the results, possible mechanisms of the formation of gizzard erosions are discussed.