The antiulcer effects of newly synthesized prostaglandin derivatives were investigated in various experimental ulcer models and on gastric secretion in rats. HK-3 and HK-4, PG $E_2$derivatives, prevented the formation of acute gastric ulcer induced by ethanol or aspirin in pylorus-ligated rats. The ulcer formation was moderately inhibited by HK-1 and HK-2, PG $F_{2{alpha}}$ derivatives, and aggravated by SK-1, SK-2 and SK-3, PG $F_{2{alpha}}$ derivatives. HK-3 and HK-4 reduced the volume, acid output and pepsin output of gastric juice in pylorus-ligated rats. The gastric perfusion with physiologic saline(pH 6.0) showed relatively constant acid secretion and indomethacin increased the acid secretion. The acid secretion was markedly decreased by PG $E_2$but PG $F_{2{alpha}}$ caused little change. Prostaglandin derivatives, especially HK-3 arid HK-4, significantly inhibited the acid secretion induced by indomethacin. The results show that, PG $E_2$ derivatives, HK-3 and HK-4, inhibit acid secretion and also have protective effects on gastric ulceration induced by ethanol or aspirin.