The kinetic properties for the uptake, storage and release of Ca 2+ from isolated mitochondria accurately predict the behaviour of the organelles within the intact cell. While the steady-state cycling of Ca 2+ across the inner membrane between independent uptake and efflux pathways seems at first sight to be symmetrical, the distinctive kinetics of the uniporter, which is highly dependent on external free Ca 2+ concentration and the efflux pathway, whose activity is clamped over a wide range of total matrix Ca 2+ by the solubility of the calcium phosphate complex provide a mechanism whereby mitochondria reversibly sequester transient elevations in cytoplasmic Ca 2+ . Under non-stimulated conditions, the same transport processes can regulate matrix Ca 2+ concentrations and hence citric acid cycle activity.