In dogs with Heidenhain pouches, the outputs of H+ elicited by endogenous gastrin were increased by administration of prednisolone intravenously; the outputs of Na+ were unchanged. An instillment of acetic acid reduced the outputs of H+ and increased the outputs of Na+, without provoking mucosal hemorrhage. When prednisolone was administered intravenously and acetic acid was administered topically, simultaneously, the outputs of H+ were higher than control values but lower than when prednisolone was given alone; the outputs of Na+ were higher than those noted after exposure of the pouches to acetic acid alone and hemorrhage occurred consistently. The results indicated that the degree of mucosal damage caused by acetic acid alone was increased by simultaneous administration of prednisolone. Nevertheless, the damaged mucosa still secreted increased outputs of H+ in response to the prednisolone.