Recent evidence indicates that prostaglandins (PGs) possess potent gastric antiulcer properties independent of their known inhibitory effects on acid secretion. We recently found that pretreatment of the stomach with pepsin protects the gastric mucosa against the damaging effect of taurocholate; this effect may be mediated by PG since it can be blocked by indomethacin, an inhibitor of PG synthesis. However, a clear association between the changes in endogenous PGs and adaptive cytoprotection by pepsin remains to be demonstrated. We now report that pepsin given intragastrically exerts a cytoprotective effect by stimulating the release of PG from the gastric mucosa. Pepsin (225 U in water, 150 U in 0.1 N HCl, or 75 U in 0.2 N HCl) was given orally to fasted rats. Fifteen minutes later, one of the following necrotizing agents was administered orally: 100% ethanol, 0.6 N HCl or 0.2 N NaOH. One hour later, animals were killed, their stomachs were removed, and the necrotic lesions were graded. Pepsin inhibited the formation of necrotic lesions in a dose dependent manner. After oral administration of pepsin in water or in 0.1 N or 0.2 N HCl, the amount of PGE and PGF2.alpha. formed by the gastric mucosa increased steadily up to 2.7-2.9-fold, and 1.9-2.5-fold of the control value respectively. The increase of PGE and PGF2.alpha. level in the gastric mucosa also follows a dose dependent manner. These observations strongly suggest that the adaptive cytoprotection by pepsin is mediated by endogenous prostaglandins. Since pepsin and HCl are natural and persistent "mild irritants" in the stomach, they may be physiologically significant in maintaining the cellular integrity of the gastric mucosa.