The activity of Mg^2+-activated adenosine triphosphatase of kidney tissue is enhanced by bicarbonate in a way suggesting an action on mitochondrial ATPase. Other anions, including S0(4)-, Cl-, N0(3)- and SCN-, affect ATP hydrolysis by all cell fractions of kidney homogenate in a manner corresponding to their position in the lyotropic series, with NO(3)- and SCN- acting as inhibitors of Mg^2+-activated ATPase.