Brief contact of the duodenal mucosa with luminal acid elicits a long-lasting bicarbonate (HCO(3)(-)) secretory response, which is believed to be the primary protective mechanism against mucosal damage. Here, we show that cGMP-dependent protein kinase type I-knockout (cGKI(-/-)) mice are unable to respond to a physiological H(+) stimulus with a HCO(3)(-) secretory response and spontaneously develop duodenal ulcerations. Smooth muscle-selective cGKI knock-in rescued the motility disturbance but not the defective HCO(3)(-) secretion. Proton-induced HCO(3)(-) secretion was not attenuated by selective inactivation of the cGKI gene in interstitial cells of Cajal or in enterocytes, but was abolished by inactivation of cGKI in neurons (ncGKI(-/-)). cGKI was expressed in the brainstem nucleus tractus solitarius that connects the afferent with the efferent N. vagus. Accordingly, truncation of the subdiaphragmal N. vagus significantly diminished proton-induced HCO(3)(-) secretion in wild-type mice, whereas stimulation of the subdiaphragmal N. vagus elicited a similar HCO(3)(-) secretory response in cGKI(-/-), ncGKI(-/-) and wild-type mice. These findings show that protection of the duodenum from acid injury requires neuronal cGKI.