Taurine and isethionic acid at concentrations ranging from 3 × 10 −5 to 6 × 10 −4 M did not change oxygen uptake of liver mitoehondria in the presence of various respiratory substrates. Taurine and isethionic acid on the contrary inhibited mitochondrial respiration in the presence of 4 mM CaCl 2. Maxive uptake of calcium and inorganic phosphate production measured after respiration experiments were not affected by taurine and isethionic acid. ATP-dependent calcium uptake in conditions of high external calcium (10 mM) was increased significantly by taurine and isethionic acid; they were also able to increase calcium binding in mediums without substrates or ATP, having external calcium concentrations of 0.5 and 0.75 mM. We can therefore suggest that taurine and isethionic acid react with calcium on the mitochondrial outer membrane and/or intermembrane spaces, increasing mitochondrial calcium-binding capacity and thus affecting mitochondrial oxygen uptake.