The effect of lansoprazole (lansoprazole) (40, 80 mg/kg) daily orally for three weeks on gastric luminal release of prostaglandin E₂ (PGE₂), pH, sodium and potassium ion contents of gastric juice, as well as plasma gastrin level was investigated on indomethacin-induced ulcer in rats. Daily administration of lansoprazole (40 or 80 mg/kg) significantly elevated the intragastric pH and K⁺ ion concentration after one, two and three weeks. Sodium ion concentration was only elevated by lansoprazole 80 mg/kg after the same time intervals. Plasma gastrin level was significantly elevated by lansoprazole in a dose dependent manner. On the other hand, lansoprazole in either doses failed to elevate the reduced level of PGE, in the gastric lumin induced by indomethacin. In conclusion, lansoprazole caused elevation in intragastric pH value, Na⁺ and K⁺ ions concentration, it also raised plasma gastrin level in a dose dependent manner, these effects would be related to inhibition of acid secretion. On the other hand lansoprazole failed to elevate the gastric luminal PGE₂ level indicating that it has no cytoprotective effect.