The effects of progesterone 10 mg/kg, estrogen 5 mg/kg and a combination of both at half these doses were studied in rats. Ulcers were induced in rats by means of various experimental models: drug-induced ulcers (aspirin or indomethacin), stress ulcer and Shay rat. The female sex hormones were found to have significant antiulcer activity in almost all the models. However, they did not affect the acidity or volume of gastric secretion in Shay's pyloric ligation model. As a result their antiulcer activity could not be explained by the effects on gastric acidity but by effects on other factors which may include enhanced mucus activity, or increase in parietal cells activity and maintenance of mucus integrity.