The role of endogenous prostaglandins as modulators of antral hormone release and gastric acid secretion was studied in the intact human stomach. The subjects (n = 9) received indomethacin prior to gastric perfusion at pH > 7 or 7 to < 2 increased twofold the basal plasma levels of somatostatin (p < 0.05) and suppressed basal and vagally stimulated gastrin release (p < 0.05) and gastric acid secretion (p < 0.05). Indomethacin prior to acidification had little effect in the basal state. Following stimulation the release of somatostatin increased, as indicated by a twofold elevation of somatostatinlike immunoreactivity in the gastric lumen (p < 0.05), but there was less inhibition of plasma gastrin (p < 0.05) and gastric acid secretion (p < 0.05) as compared to acidification alone. During alkaline gastric perfusion, indomethacin increased circulating somatostatin (p < 0.05) levels without affecting plasma gastrin or gastric acid. Indomethacin given against intravenously infused somatostatin (0.1 μg•kg-1·h-1) partially reversed the inhibited gastrin response to vagal stimulation without affecting somatostatin-suppressed gastric acid secretion. The effects of indomethacin against pentagastrin were marginal. In conclusion: Gastric acidification in man stimulates plasma release of somatostatin in parallel to suppressing gastrin release and gastric acid secretion. Endogenous prostanoids participate to regulate antral hormone interactions and may have dual actions on antral somatostatin, as negative modulators of release and as mediators of somatostatin effects on the gastrin cell. It is suggested that an unrestricted release of antral somatostatin is reflected in the gastric lumen rather than in the circulat