The coupling factor of oxidative phosphorylation from beef heart mitochondria obtained as a "latent F1," exhibits negligible levels of ATPase activity, contains stoichiometric amounts of the specific F1 inhibitor protein, and is stable to incubation at low temperature (Adolfsen, R., McClung, J.A., and Moudrianakis, E. N. (1975) Biochemistry 14, 1727-1735). Incubation of the latent F1 enzyme at 60 degrees C activates its ATPase activity. We show in this paper that regulation of the interaction of the inhibitor protein with the latent F1 enzyme can be accomplished under more physiological conditions. At 37 degrees C, variations in the proton concentration led to changes in the degree and extent of activation of the enzyme, with maximal activation rates occurring after preincubation at pH 9.6. The energy for the pH 9.6-induced activation process (12.1 kcal/mol) was similar to that reported for the dissociation of the inhibitor protein from the membrane-bound F1 enzyme in energized mitochondria (Gomez-Fernandez, J. C., and Harris, D.A. (1978) Biochem. J. 176, 967-973). The rates of activation were higher in the presence of 5 mM ATP and inhibited by the presence of Mg2+, suggesting the existence of a specific binding site for Mg2+ between the inhibitor subunit and the F1 enzyme. A model is presented in which the activation of the latent F1 enzyme is brought about by a rapid titration of positively charged amino acid residues on the inhibitor subunit, followed by a slow release of a tightly bound Mg2+ atom. This model predicts that the initial event leading to the appearance of ATP synthesis is the deprotonation of the inhibitor subunit and that the onset of ATPase activity in mitochondria is due to sequestering of the available free Mg2+.