In the present study, a sequential observation of the gastric lesions or ulcers in rats weighing from 100 g was made with an endoscope. It was found that bleeding, edema, hyperemia, fur of ulcer and convergences of the mucosal folds in the stomach could be observed in rats with a SES-1717S (phi = 1.7 mm) or SES-2217S (phi = 2.2 mm) endoscope. Furthermore, we have devised a method which we named the "sohmen method" for measuring ulcer-sizes with an endoscope. The ulcer-size was compared with 5 mm lengths of sohmen (Japanease vermicelli) which were inserted through the sheath of the endoscope into the stomach. Therefore, the ulcer-size could be accurately measured by this method. By this observation method, it is possible to observe repeatedly over a long term the changes in gastric lesions or ulcers, to measure changes in ulcer-sizes and to evaluate the relapse or recurrence of ulcers in the same rats.