To elucidate the effect of aging on the vitamin B12 (B12) transport in enterocytes, young and old (3-4 months and over 1.5 years, respectively) female rats were studied. Two units of rat intrinsic factor (IF), saturated with 57Co-labeled cyanocobalamin were orally administered, and the amount of B12 absorbed into each subcellular fraction of enterocytes was assayed. Concentration of endogenous B12 in each subcellular fraction was also studied. Absorption of radioactive B12 in the lysosomal fraction was maximum between 2 and 4 hr in each age group. In the older rats, the amount absorbed was lower than in the young rats. The older rats showed a significantly lower value of endogenous B12 in the mitochondria. It has already been reported by us that there exist two B12 binders in enterocytes: lysosomal and microsomal binders. The concentrations of lysosomal and microsomal binders as well as B12 uptake in the mitochondria were significantly lower in the older rats than in the young rats. These data might help to explain the lower B12 absorption in the lysosomal fraction and lower B12 contents in the mitochondrial fraction.