The present study investigated the preventive and curative effects of prostaglandins (PGs) on gastric ulcer in rats induced by physical or psychological stresses; some rats were electrically shocked, while others were exposed to affective stimuli arising from the shocked animals. The synthetic PGs dimethyl-PGE2 and rioprostil were administered orally, and their preventive effect on gastric ulceration was evaluated by determining the incidence and the ulcer index of lesions. The curative effect of drugs on ulcer healing was evaluated by determining a time-dependent change in the mucosal surface of the stomach with an endoscopic technique. Oral administration of dimethyl-PGE2 or rioprostil (25 and 50 micrograms/kg) prevented gastric ulceration significantly. Oral administration of these drugs (50 micrograms/kg, twice per day) significantly promoted the healing process of lesions 24 and 36 hr after termination of stress loading. The present results give direct evidence of the curative effect of PGs on stress ulcers and suggest that application of the endoscopic technique to the pathology of the rat's stomach may be a substantial aid in the preclinical evaluation of antiulcer drugs.