Calcium distribution in dendritic spines of the dentate fascia was studied as a function of age with the oxalate-pyroantimonate precipitation technique. In postnatal ages P3, P9, P24 and P30 spines were analyzed as to the presence of the spine apparatus (SA) and as to the presence of Ca2+ deposits within the SA and within the spine cytoplasm. The percentage of spines with SA-containing precipitates declined significantly between P3 and P24. Conversely, the percentage of spines with precipitates in the spine cytoplasm was significantly increased by P24. In the absence of an SA loss, this result suggests an age-related decrease in the Ca2+-sequestering capacity by the SA. These parameters were improved by P30 so that they approximated the values of P3. Such a seeming amelioration could be attributed to the fact that the mortality rate in rats sharply increases by P24, so that animals surviving this age represent a selected population in which a compensatory growth of spines has occurred and has secured functionally valid connections.