Ba++ added to the serosal solution bathing a resting frog stomach increased transmucosal resistance and induced acid secretion. The increase in acid secretion was associated with an increased leak of histamine from the mucosa into the serosal solution. Pretreatment with burimamide inhibited the secretory response to Ba++. After a pulse treatment with Ba++ (2 mM for 5 min), the effects on resistance were transient but the effects on acid secretion were sustained. A pulse treatment with histamine also led to a sustained increase in acid secretion, and this increase was blocked by burimamide. It is concluded that Ba++ releases histamine which then stimulates acid secretion.