45Ca2+-40Ca2+ exchangeability of 45Ca bound to the calcium transport sites of unphosphorylated sarcoplasmic reticulum Ca2+-ATPase at equilibrium has been found to be heterogeneous: Half of the bound calcium is [Ca2+]-dependent in a slowly exchangeable (k less than 0.3 s-1), "occluded" state in the Ca2+-ATPase, and the other calcium is [Ca2+]-independent in a rapidly exchangeable (k approximately 0.3 s-1), "unoccluded" state (Nakamura, J. (1986) Biochim. Biophys. Acta 870, 495-501). In this paper, the two different forms of exchangeable calcium were studied after phosphorylation of the enzyme by ATP without added Mg2+ at pH 7.0 and 0 degree C. By the phosphorylation, the degree of the occlusion became higher (k less than 0.03 s-1). The unoccluded calcium was, however, not significantly affected. The more highly occluded calcium exchanged at the same rate as the decay rate of the phosphoenzyme (EP) in the steady state at a ratio of about 1:1. The occluded calcium was relieved by dephosphorylation of EP by ADP. These results suggest that 1 mol of ADP-sensitive EP more highly occluded 1 mol of calcium, already occluded before phosphorylation. After transformation of ADP-sensitive EP to its ADP-insensitive form by the addition of 20 mM Mg2+ at pH 8.8, the unoccluded calcium was rapidly (k = 0.1-0.3 s-1) released from the transformed EP. However, the occluded calcium was maintained in an occluded state in which the calcium was slowly (k approximately 0.01 s-1) released from the EP without exchange. The results suggest that calcium occlusion in the ADP-sensitive EP is not relieved by the loss of ADP sensitivity of the EP itself.