The rapid single intravenous injection of the cholecystokinin preparation Cecekin (Vitrum) was found to inhibit the responses of Heidenhain pouch dogs to stimulation by gastrin extract or synthetic gastrin-like penta-peptide at both submaximal and supramaximal dose rates, to histamine, to the stable cholinergic drug Amechol, and to feeding a meat meal. The pattern of inhibition suggested that the inhibitor effect was exerted in the region of the acid secreting cell. Pure cholecystokinin was capable of inhibiting the acid response of the Heidenhain pouch dogs to histamine. Although both pure secretin and pure cholecystokinin have now been demonstrated to be possible inhibitor agents arising from duodenal mucosa, it is likely that endogenous duodenal acidification releases a further as yet unidentified humoral inhibitor agent, or that secretin and/or cholecystokinin may act in collaboration with each other or a further inhibitor agent. In dogs with both a Heidenhain pouch and a simple fistula into the normally vagally innervated gastric remnant and in which antrectomy had been performed, a single rapid intravenous injection of the Cecekin had no effect on the Heidenhain pouch responses to the continuous intravenous infusion of histamine. The acid output from the main gastric remnant, however, was increased by the Cecekin injection. Several hypotheses to account for the difference in behaviour in the two preparations are discussed. The possibility is raised that Cecekin contains some gastrin-like activity.