The role of changes in cytosolic concentrations of Ca2+(here, Ca2+ refers exclusively to free calcium ions) in the regulation of key cellular systems, such as muscle contraction, secretion, and glycogen breakdown by hormones and other extrinsic factors, has long been recognized (55, 56, 275). Considerable research effort has been focused on the mechanisms by which the cytosolic concentration of Ca2+ is controlled. It has been established that alterations in the activities of Ca2+-transporting systems in both the plasma membrane and microsomal membranes occur, and it is widely accepted that inositol trisphosphate acts as the second messenger through which hormones mobilize calcium from intracellular microsomal stores (30, 95). In contrast, general agreement that Ca2’is important in the regulation of mitochondrial metabolism has only developed over the last 10 years. Extensive studies in the 1960s and 1970s had revealed that mitochondria, at least from vertebrate sources, contained very active systems within their inner membranes that allowed the specific transfer of Ca2+ both into and out of mitochondria (for reviews see Refs. 2, 56, 249). However, it was widely assumed that these systems were concerned primarily with the regulation of cytosolic Ca2+ concentra-