In the healing test of acetic acid ulcers in rats with limited food-intaketime, Z-1 03 given, p.o., at doses of 3 and 10 mg/kg, twice a day, for 14 consecutive days from the day after acetic acid injection not only reduced the size and depth of the ulcers, but also promoted the regeneration of the defective mucosa. In the hydrocortisone-induced relapse test of acetic acid ulcers in rats with limited food-intake-time, Z-103 given, p.o., twice a day, at doses of 3 and 10 mg/kg for 20 consecutive days from the 40th day after the acid injection strongly prevented the exfoliation of the regenerated mucosa. Cimetidine (100 mg/kg × 2/day, p.o.), like Z-103, showed a marked relapse-preventive action in addition to the healing-promoting action. However, it was more effective on the healing. Gefarnate (300 mg/kg × 2/day, p.o.) markedly reduced the size and depth of the ulcers and strongly prevented the steroid-induced relapse, but showed no apparent effect on the regeneration of the defective mucosa. These results suggest that Z-103 may be a new therapeutic agent sharing both healing-promoting and relapse-preventive actions on gastric ulcers.