Changes in synaptic density in various brain regions were assessed among different age groups of rats maintained in ordinary small cages, as determined by synaptophysin assay. The synaptophysin content in hippocampus decreases as early as in the adult stage. The most remarkable decrement occurs in occipital cortex. In other regions, synaptophysin contents decrease in senescence to 60-77% of the respective peak values during young and adult stages. The other rat group reared under enriched environment in a large cage until 30 months of age was examined for synaptic density, and was revealed to maintain the similar levels as in young, or even higher levels in frontal, temporal, entorhinal cortices and hippocampus. These results indicate that the synaptic density in cerebrum decreases in senescence and this decrease can be prevented by rearing under enriched environment.