The hippocampus is associated with learning and memory function and shows neurochemical changes in aging processes. Calbindin D-28k (CB) binds calcium ion with a fast association rate. We examined age-related changes in CB immunoreactivity and its protein level in the gerbil hippocampus during normal aging. In the hippocampal CA1 region (CA1) and CA2, CB immunoreaction was found in some neurons in the stratum pyramidale (SP) at postnatal month 1 (PM 1). CB immunoreactivity in neurons was markedly increased at PM 3. Thereafter, CB immunoreactivity was decreased with time: CB-immunoreactive ((+)) neurons were fewest at PM 24. In the CA3, a few CB(+) neurons were found only in the SP at PM 1 and in the stratum radiatum at PM 18 and 24. In addition, mossy fibers were stained with CB at PM 1. CB immunoreactivity in mossy fibers was markedly increased at PM 3, thereafter it was decreased with time. In the dentate gyrus, many granule cells (GC) in the granule cell layer were stained with CB at PM 1. CB immunoreactivity in GC was markedly increased at PM 3, thereafter CB immunoreactivity was decreased with time. In Western blot analysis, CB protein level in the gerbil hippocampus was highest at PM 3, thereafter CB protein levels were decreased with time. This result indicates that CB in the gerbil hippocampus is abundant at PM 3 and is decreased with age.