Bicarbonate, maleate, and phosphate were shown to modulate adenosinetriphosphatase (ATPase) activity in coupling factor 1 from chloroplasts. Kinetic analysis of the changes in the ratio between the apparent Km with and without effectors indicated that the stimulation of the activity by bicarbonate was a result of a decrease in the Km for MnATP2-. The inhibition by phosphate resulted from a decrease in the K, for free ATP as a competitive inhibitor at pH 8. The effectors did not change at this pH. However, at pH 6.5, both Km and Vmax of ATPase activity with MnATP2- were changed by maleate, yet the mode of inhibition by free ATP remained unaltered. In addition to decreasing the Km, bicarbonate induced a 10-fold decrease in the Kd for binding of Mn2+ at the two tight binding sites in the presence of ATP at pH 8. At pH 6.5, maleate also decreased both the Km for MnATP2- and the Kd for Mn2+ binding. A decrease in the Km of a substrate induced by an effector is likely to be a result of a decrease in the binding constant of the substrate. Therefore, these results are in harmony with the suggested assignment of the two tight binding sites of Mn2+ at the active sites of the enzyme.