The present study examined whether the central neurons are involved in the stimulatory action of gastrin on the secretion of gastric acid. Gastrin (20 microg), which was examined and ascertained to induce a marked increase in gastric acid secretion in gastric-lumen perfused rats, was intravenously injected in Wistar rats under anesthesia with pentobarbital sodium. In the experiments, 1 h after injecting gastrin, rats were perfused and fixed, the brain was removed and sectioned at 40 microm thickness. Every fourth section was treated with anti-c-Fos antiserum, and c-Fos protein was immunohistochemically stained using the avidin-biotin complex method. It was found that c-Fos protein was expressed in neurons of the lateral habenular nucleus, the central nucleus amygdala, the lateral parabrachial nucleus in the pons, and the complex area of the nucleus of the solitary tract and the dorsal motor nucleus of the vagus nerve in the medulla oblongata. The control rats were injected with saline solution, and the brain sections were processed similarly as described above. c-Fos protein was expressed in few neurons in the nuclei above in the control rats. These results suggest that gastrin released into the circulation might stimulate central neurons which, in turn, may relate to the control mechanism for the secretion of gastric acid.