The effect of varying Nao and Nai on Ca efflux while maintaining the ratio Nao/Nai constant was explored in squid giant axons dialyzed with and without ATP. In the absence of ATP, the Ca efflux increased 3.4 +/- 0.2-fold when the Nao/Nai concentrations were reduced from 440/80 to 110/20 mM. In the presence of ATP a similar change did not have an appreciable effect. The inhibition of Ca efflux produced by Nai was studied in the presence and in the absence of ATP. In the absence of ATP, inhibition is very marked and is reminiscent of a unimolecular noncompetitive reaction (inactivation constant [KI] of 34 +/- 5 mM of Nai) whereas in the presence of ATP, the slight inhibition observed indicates that ATP probably increases the KI to 200mM. From the inhibition of the Ca efflux produced by Nai in the presence or absence of ATP a curve describing the dependence of Nai of the ATP-promoted fraction of Ca efflux was constructed. The effect of Nao on the Ca efflux was studied as a function of [Na]i: at low Nai, an activation constant (KA) of 41 mM for Nao was obtained either in the presence of in the absence of ATP. As the intracellular Na is increased in the presence of ATP, Nai seems to have no effect on the apparent half-activation constant. However, in the absence of ATP, the KA for activation increases along a sigmoid curve reaching a value of 112 mM at 100 mM Nai. It is concluded that the Ca efflux system uses the energy of the Na electrochemical gradient. The action of Nai appears to be such that the interaction of a single Na+ is sufficient to block Ca extrusion whereas several Naps externally are necessary to activate Ca extrusion.