The incidence of gastric cancer differs greatly by the site of the stomach, and cancer has predilection for the area covering the lesser curvature. Some gastric diseases (e.g., cancer and atrophic gastritis) increase in the old age. Therefore, I have coducted a series of studies of LDH isozyme of astric mucosa in Wistar rats. The pattern of LDH isozyme of gastric mucosa separated by means of agar gel electrophoresis in the adult rat differs among topographically divided sections of mucosa. The heterogeneity of LDH is explained by the synthesis of two types of monomers, namely, H and M types. I found more of M type LDH in the lesser curvature, while more of H type LDH in the greater curvature. The changes of the isozyme pattern were observed through the aging process. It was found that the most important farction of pyloric mucosa on the lesser curvature was the slowest fraction, LD-1, and the pattern did not change by aging. Nor was there any difference in the pattern of the lesser curvature on the cardia side. On the other hand, in the mucosa of the greater curvature of newborn rats LD-1 occupied 73%, resembling the pattern of the lesser curvature, but LD-1 tended to decrease and the fast moving isozyme increased gradually through the aging process. These results indicate that M type LDH is found abundantly in the mucosa of the lesser curvature, while in the mucosa of the greater curvature the predominant pattern is converted gradually from M type LDH to H type LDH. The effect of various agents on LDH isozyme of gastric mucosa was also studied. The changes of isozyme pattern were also observed by the continuous administration of atropine and cortison. The isozyme of neighboring mucosa of experimental ulcer showed an increase of LD-1 fraction. From these findings, it is assumed that the metabolic patterns in the lesser curvature are adapted to anaerobic conditions, and these patterns resemble those of cancerous tissue. In addition, the greater curvature gradually becomes adaptable to aerobic condition during the aging process. Therefore, the differences in LDH isozyme pattern observed between the mucosa of the lesser and greater curvatures would reflect not only the morphological difference but also the difference in the specificity of metabolism accounts for more frequent development of gastric cancer on the lesser curvature.