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Inside-out vesicles from the membranes of dog erythrocytes were obtained by the method of Lew & Seymour (1982) for study of Ca movements. In the absence of ATP, 45Ca accumulation by the vesicles was inhibited by external Na and stimulated by internal Na. The presence of either MgCl2, quinidine sulphate, or LaCl3 in the incubation medium inhibited 45Ca accumulation in the absence of ATP. The release of 45Ca from 45Ca-loaded vesicles was specifically promoted by Na+ in the absence as well as in the presence of ATP. The accumulation of 45Ca by vesicles was stimulated by ATP and the effect of ATP was entirely dependent on the presence of Mg. The Mg- and ATP-dependent 45Ca accumulation was stimulated by the presence of either K or Na in the medium, was hyperbolically activated by increasing the Ca2+ concentration in the medium, was stimulated by calmodulin and inhibited by orthovanadate (10(-4) M) or LaCl3 (10(-3) M). The data demonstrate the presence of two mechanisms for controlling Ca movements in inside-out vesicles from dog erythrocyte membranes, a Na-dependent one similar to the Na-Ca exchange described for squid axons and cardiac muscle and a Ca pump utilizing ATP with characteristics similar to those described for human erythrocytes and squid axons.