Calcium membrane extrusion in squid axons is mediated by two mechanisms able to regulate the Ca2+ with different degrees of efficiency. The ATP driven uncoupled Ca pump with its high affinity for Ca2+i will work primarily to balance the physiological Ca leak, hence controlling the [Ca2+]i at rest. The Na-Ca exchange system, although less important in the maintenance of the resting Ca2+i owing to its large capacity of transport (outward and inward), will certainly be important in physiological and unphysiological conditions in which relatively high [Ca2+]i concentrations are attained.