The effect of ATP on calcium binding of the Ca2+-ATPase of the sarcoplasmic reticulum has not been clarified. By comparing the calcium dependence of the ATPase activity and of phosphorylation of the ATPase molecules with that of calcium binding in the absence of ATP, we show the existence of two types of regulatory site of the enzyme molecules at which ATP binding variously improves the calcium binding performance of the molecules depending on the aggregation state of the molecules and pH; the two regulatory sites bind ATP at submillimolar (0.25 mm) and millimolar (5 mm) ATP, respectively. The results are discussed based on a model of two conformational variants (A and B forms) of the chemically equivalent ATPase molecules (Nakamura, J., and Furukohri, T. (1994) J. Biol. Chem. 269, 30818-30821). For example, in the sarcoplasmic reticulum membrane at pH 7.40, submillimolar ATP converted the calcium binding manner of the A form from noncooperative (Hill number (n(H)) of approximately 1) to cooperative (n(H) approximately 2), concurrent with a decrease in the apparent calcium affinity (K(0.5)) from 2-6 to 0.1-0.3 microm. The binding of the A form became almost the same as that of the B form (n(H) approximately 2, K(0.5) approximately 0.2 microm), which was not affected by ATP. Millimolar ATP further decreased the K(0.5) of the cooperative binding of the two forms to approximately 0.05 microm. Regulation of the calcium binding performance by ATP is discussed in terms of monomeric and oligomeric pathway models.