Adsorption of B_12,with and without gastric mucosa extract (GME), was studied on intestinal mucosa homogenate from rats at various stages of growth. The development of the productive activities of gastric HCI and pepsinogen was also investigated. The intestine from newborn animals showed extremely high adsorption of B_12 in the absence of GME. The GME-independent B_12 adsorption fell rapidly with time, approaching adult levels at about 20 days. Stimulation of B_12 adsorption by adult GME and gastric HCI secretion reached adult levels at about 20 days. On the other hand, intrinsic factor formation and pepsinogen formation did not attain adult levels until about 30 days. The histologic investigations on the gastric gland and ileal villus confirmed above-mentioned experimental results. The intrinsic factor-dependent B_12 absorption became the predominant system at about 30 days after birth. (Received July 8,1971)