Antiplatelet aggregators such as sulfinpyrazone (10 mg/kg), ticlopidine (200 mg/kg), and pyridine (0.1 mg/kg) significantly inhibited the degree of gastric ulceration in Sprague-Dawley rats subjected to cold restraint stress. The inhibition of platelet aggregation and that of microthrombus formation were evident in rats given antiplatelet aggregating drugs and exposed to cold stress. Therefore, platelet aggregation does play an important role in stress ulcer formation, and drugs which inhibit this aggregation should be given greater attention as to possible clinical application for treatment of stress ulcers.