The stomach cancer and atrophic gastritis show the greatest rate of occurrence in old ages, and the variations of serum lactic dehydrogenase (LDH) isozyme pattern were already reported in patients with malignant diseases and acute hepatitis. The authors, therefore, studied on LDH isozymes of gastric mucosa of surgically resected stomachs of human and rat. The results were as follows: 1) The LDH isozyme in lesser curvature of human stomach showed the increased M type of LDH and decreased H type, the pattern in greater curvature was opposed to lesser curvature in chronic gastritis and gastric ulcer. 2) The LDH isozyme pattern of cancer tissue of stomach showed the increased M type of LDH, and the increased M type and decreased H type of LDH were estimated even in greater curvature without infiltration of stomach cancer. 3) The LDH isozyme pattern of gastric mucosa of Wistar rat showed the difference between lesser and greater curvatures. In lesser curvarure, the largest fraction of LDH isozyme was LD1 (the slowest moving fraction), and the other fractions were very small. On the other hand, greater curvature showed five fractions of LDH in adult rat. 4) The variations of LDH isozyme pattern by aging were not observed in lesser curvature, but in greater curvature the variations were significant. The greater curvature of new-born rat showed increased LD1 (73%), and other fractions were demonstrated very small amount or not. The pattern at 2 weeks after birth showed the decrease of LD1 (56%) and the increases of other fractions. The amount of LD1, thereafter, decreased gradually by aging, and other fractions increased. Young adult (1 month after birth) rats demonstrated five fractions of LDH isozyme, and the amounts of LD1, LD2 and LD3 in adult rat were almost equal. In old rat (over 1 year), the amounts of four fractions except LD5 were almost equal, and LD5 was significant fraction. 5) These data mean to change the biochemical nature of greater curvature from anaerobic condition to aerobic by aging process. Then, we speculated that these variations of LDH isozyme pattern of gastric mucosa were correlated with the differences of the carcinoma rate between lesser and greater curvatures.