The effect of IMX on histamine-stimulated gastric secretion has been studied in dogs equipped with gastric cannulas. After determining the histamine dose response, IMX, histamine, and IMX plus histamine were given by a constant intravenous infusion for 3 h, and acid was collected at 15-min intervals. In a separate series of experiments a single dose of 100 mg of db-cAMP was given 1 h after initiation of infusion of IMX, histamine, and histamine plus IMX. Results show that IMX, a potent inhibitor of phosphodiesterase, is a weak stimulant of acid secretion, and augments histamine-stimulated secretion. However, during stimulated acid secretion, there were no significant changes in tissue cAMP levels, Administration of exogenous dib-cAMP did not produce any significant changes in acid secretion in dogs, but did contribute to elevated cAMP levels.