Fragmented sarcoplasmic reticulum isolated from skeletal muscle of the rabbit has a cation-binding capacity of about 350 microeq/g of protein at neutral pH. The same binding sites bind Ca, Mg, K, and H ions and, consequently, the selective binding of Ca induced by ATP releases an amount of the other cations equivalent to the Ca taken up. At pH values below 6.2, an increasing number of binding sites are associated with H(+), and ATP induces exchange of Ca mostly for H(+). At pH values above 6.2, the binding sites exist in the form of Mg and K, and Ca is bound in exchange for these cations. The total bound Ca + Mg + K, expressed in microequivalents of cations bound per gram of protein, is approximately constant at various pCa values, which indicates a stoichiometric exchange of Ca for the other cations. To accomplish the same degree of exchange of Ca for other cations bound, in the absence of ATP, concentrations of free Ca(++) of about 1000-fold higher than those needed in the presence of ATP are required in the medium. We cannot distinguish between a mechanism whereby Ca actively transported into a compartment of the microsomal vesicles containing also the binding sites is bound passively to these sites in exchange for Mg, K, and H and another in which ATP selectively increases the affinity of surface-binding sites for Ca. Irrespective of the mechanism of accumulation, the Ca retained does not contribute to the activity of the cation in the membrane fraction. Caffeine (10 mM) has no effect on the binding of Ca, but releases a more labile fraction of Ca, which presumably accumulates in excess of the bound Ca. Procaine (5 mM) antagonizes the effect of caffeine. Acetylcholine and epinephrine have no effect on the binding of Ca.