Stimulation of the rat liver mitochondrial inner-membrane anion-conducting channel by aeration is dependent on the concentration of Ca2+ ions in the assay medium. Ca2+ activates anion conduction in both aerated and non-energised mitochondria but acts over a wider concentration range *+ and produces a greater increase in anion-conductivity in aerated mitochondria. EGTA reverses Ca stimulation but takes several seconds to act, *+ indicating slow release of Ca from the activation site possibly on the matrix side of the inner mitochondrial membrane. It is suggested that this channel may respond to hormone-induced changes in cytosolic Caz+ concentration.