Based on evidence that high Mg2+ counteracts age-related declines in synaptic plasticity, and that aged rat hippocampal neurons exhibit prolonged Ca2+-dependent K+ currents, it is proposed that an underlying cause of altered Ca2+ homeostasis during brain aging may be an increased membrane conductance to Ca2+. An apparent Ca2+-mediated inactivation of Ca2+ current, which was recently described in hippocampus, could account for some of the contradictions in the literature.