OBJECTIVE Secretin is an inhibitory hormone of gastric acid secretion. However, its inhibitory mechanism has not been well understood. Possible roles of both somatostatin and prostaglandins were investigated. METHODS Totally isolated rat stomachs were vascularly perfused with Krebs-Ringer buffer containing 50 mumol/L isobutyl methylxanthine at 1.4 mL.min-1. Gastric lumen was perfused with 0.15 mol/L NaCl at 1.0 min.min-1. Effect of secretin in three different doses given intra-arterially on basal acid secretion and acid secretion stimulated by pentagastrin was studied. To determine roles of somatostatin and prostaglandins in the secretin-induced inhibition, an antisomatostatin serum and indomethacin were tested, and both somatostatin and prostaglandin E2 concentrations in portal venous effluent were determined by radioimmunoassay. RESULTS Both basal- and pentagastrin-stimulated acid secretion were significantly inhibited by secretin. The inhibition was completely reversed by either indomethacin or antisomatostatin serum. Secretin significantly increased concentrations of both somatostatin and prostaglandin E2. Although indomethacin blocked the increase in prostaglandin E2, secretin-induced increase in prostaglandin E2 was not affected by antisomatostatin serum or was indomethacin influenced by somatostatin level. Finally, the inhibition by somatostatin of acid secretion was not affected by indomethacin. CONCLUSIONS The inhibition of gastric acid secretion by secretin in rats is mediated by simultaneous releases of both somatostatin and prostaglandin E2, which independently inhibit gastric acid secretion.