The ability of isolated mitochondria to accumulate very large amounts of Ca++ by an active transport mechanism was discovered by V a s i n g t o n and M u r p h y in 1961 [1]. The accumulation process was found to be blocked by respiratory inhibitors and by uncouplers of oxidative phosphorylation, and thus was completely dependent on electron transport and the energy coupling mechanism of oxidative phosphorylation. Significantly, oligomycin does not inhibit Ca++ uptake under these conditions. Later it was found that during the accumulation of Ca++, inorganic phosphate disappears from the medium and also accumulates in mitochondria in amounts that are stoichiometrically related to the Ca++; the molar ratio with which Ca++ and phosphate are accumulated is almost exactly that of Ca++ hydroxyapatite. The amounts of Ca++ and phosphate that accumulate may exceed 200 times the normal mitochondrial content; such accumulation is called “massive loading”. The accumulation of Ca++ and phosphate can also be driven by ATP hydrolysis, and in this case it is not sensitive to respiratory inhibitors, but is inhibited by uncouplers of oxidative phosphorylation and by oligomycin. Ca++ uptake is evidently associated with the dinitrophenolsensitive formation of the first high energy intermediate formed during electron transport, before the point of action of oligomycin.