THE soluble adenosine triphosphatase (ATPase) from mitochondria1–3 is activated by several bivalent metal ions, and the pattern of activation is essentially the same whether the enzyme is acting on ATP (Fig. 1) or ITP (Fig. 2). It is clear that the ionic radius is the chief factor determining the ability of a bivalent ion to activate this enzyme. This indicates that ligand field effects are unimportant and that the metal ion remains in a coordination field similar to that of the aquo-ion, that is, an octahedral system which does not involve coordination to any high or low field ligands. In the range from Mg2+ to Ca2+ the only ion which fails to activate the enzyme is Cu2+. This is not because Cu2+ ions have a secondary effect as inhibitors, as 2.5 mM Cu2+ ions produce only 45 per cent inhibition in the Mg2+ activated system, which is compatible with competition between the ATP-Mg2+ complex and an inactive ATP-Cu2+ complex, but not with the total inhibition by Cu2+ ions which would be necessary to mask any activation they produced. Cu2+ may fail to activate because it does not form regular octahedral complexes.