A chronically indwelling gastric fistula was implanted into rats. To collect gastric acid, a collection system was coupled to the fistula. This system operated by infusing physiological saline directly into the stomach of unanesthetized rats; the acid content of the perfusate was immediately determined by automatic titration machinery. Using this technique, a value of 3.25 ueq H+/2 min was obtained as the basal gastric section. For a sham-fed test meal of 2 ml of 36% sucrose, the stimulated phase of gastric acid secretion consisted of a dynamic response (peak 200% of basal) with a brief latency (< 2 min) and short duration (< 24 min). This response to sham feeding was apparently a cephalic response, since direct infusion of a similar test meal into the stomach did not alter basal gastric acid output. Experience with the perfusion-collection technique indicated the procedure was apparently a sensitive nondisruptive method for chronic gastric acid collection from unanesthetized animals.