We have previously identified cells containing the enzyme nitric oxide synthase (NOS) in the human gastric mucosa. Moreover, we have demonstrated that endogenous and exogenous nitric oxide (NO) was shown to decrease histamine-stimulated acid secretion in isolated human gastric glands. The present investigation aimed to further determine whether this action of NO was mediated by the activation of guanylyl cyclase (GC) and subsequent production of cyclic GMP (cGMP). Isolated gastric glands were obtained after enzymatic digestion of biopsies taken from the oxyntic mucosa of healthy volunteers. Acid secretion was assessed by measuring ¹⁴Caminopyrine accumulation and the concentration of cGMP was determined by radioimmunoassay (RIA). In addition, immunohistochemistry was used to examine the localization of cGMP in mucosal preparations after stimulation with the NO-donor S-nitroso-N-acetylpenicillamine (SNAP). SNAP (0.1 mM) was shown to decrease acid secretion stimulated by histamine (50 µM); this effect was accompanied by an increase in cGMP production, which was histologically localized to the parietal cells. The membrane permeable cGMP analogue dibuturyl-cGMP (db-cGMP; 0.1-1 mM), dose-dependently inhibited acid secretion. Additionally, the effect of SNAP was prevented by pre-incubating the glands with the GC inhibitor 4H-8-Bromo-1,2,4- oxadiazolo[3,4-d]benz[b][1,4]oxazin-1-one (NS 2028) (10 µM). We therefore suggest that NO in the human gastric mucosa is of physiological importance in regulating acid secretion. Furthermore, the results show that NO-induced inhibition of gastric acid secretion is a cGMP-dependent mechanism in the parietal cell involving the activation of GC.