Oral administration of HCl (0.2 N, 0.4 N, 0.6 N, 0.8 N) solution produced congestive or hemorrhagic gastric mucosal lesions in rats. Lesion areas were differentiated into two areas (i.e., brown area and black area) macroscopically. Then, fine observations were performed on two areas using a computer image processing system (CIPS) and histological techniques. Cell necrosis from the mucosal surface to the middle mucosal layer were observed in the brown area, and gastric mucosal necrosis that reached to a deeper layer were observed in the black area. These results indicate that histological degree of lesion may be reflected in the macroscopic changes. Concerning the concentration of HCl, over 0.4 N HCl solution induced mucosal lesions, but HCl solution (0.4 N HCl) only produced a brown area. Maximum changes in the mucosal surface were observed at 30 min to 1 hr by 0.6 N HCl and at 15 to 30 min by 0.8 N HCl. Therefore, the best condition for evaluating the preventive effects of anti-ulcer agents is 30 min after treatment by 0.6 N HCl solution and 15-30 min after treatment by 0.8 N HCl solution. These results suggest that this technique may provide both quantitative and qualitative information on histological changes.