A previous communication (Fagian, M. M., Pereira da Silva, L. and Vercesi, A. E. (1986) Biochim. Biophys. Acta 852, 262-268) indicated that intramitochondrial calcium inhibits oxidative phosphorylation by decreasing the availability of adenine nucleotides to both the ADP/ATP translocase and the F0F1-ATP synthase complex. In this work we analyzed the interactions of calcium-nucleotide and magnesium-nucleotide complexes with the ATP synthase during catalysis of ATP in equilibrium with [32P]Pi exchange and net synthesis of ATP by submitochondrial particles. Concerning the ATP in equilibrium with [32P]Pi exchange reaction, calcium was ineffective as divalent cation when assayed alone. Furthermore, the addition of calcium increased the magnesium concentration required for half-maximal activation of the exchange, without changing Vmax. With respect to net ATP synthesis, the inhibition by calcium was shown to be due to formation of the CaADP- complex, which competes with MgADP- for the active site of the F0F1-ATP synthase. Moreover, ATP hydrolysis was competitively inhibited by CaATP2-, showing that calcium is able to interact with the enzyme in both forward and backward reactions in the same manner. That high calcium concentrations are required for significant inhibition of ATP synthesis indicates that this inhibition is relevant under conditions in which cytosolic calcium concentrations rise to pathological levels. Therefore, this mechanism may be responsible, in part, for the decrease in cellular ATP content that has been observed to occur when calcium accumulates in the cytosol.1020101-6