Mitochondrial ATPase binds ADP tightly, forming inactive complexes. Dissociation of the complexes is blocked by azide. Azide inhibits neither ATP hydrolysis at high δμH + nor ATP-dependent NAD + reduction, provided that the ADP concentration is higher than 10 - 4 M. At lower ADP levels, azide is inhibitory. These data suggest that in the presence of ATP δμH + prevents the fromation of one of the inactive complexes. In the absence of ATP at high δμH + , azide-sensitive complexes are not formed at any ADP concentrations tested (5 x 10 - 7 -5 x 10 - 4 M). The inactive E.ADP complexes can play a significant role in the regulation of ATPase in mitochondria, preventing futile ATP hydrolysis at low δμH + .