1. Beef heart mitochondrial ATPase, in both the membrane-bound and isolated form, contains tightly bound ATP and ADP. Each mol of ATPase contains about 2.2 mol ATP and 1.3 mol ADP. 2. In the absence of ATPase activity, these nucleotides exchange only slowly with nucleotides in solution. The exchange rate is increased during coupled ATPase activity, but not when the ATPase is uncoupled. 3. Oligomycin and dicyclohexylcarbodiimide inhibit exchange of the bound nucleotides, as does the ATPase inhibitor protein, although in each case some residual exchange occurs. Aurovertin, although inhibiting phosphorylation, does not inhibit the exchange. This is discussed in terms of the reversibility of these inhibitors. 4. The stimulation of exchange seen during coupled ATPase activity requires energisation of the ATPase molecule. Using the exchange reaction as a probe of energisation, it is deduced that energy can be transferred between different ATPase molecules. 5. It is proposed that coupled ATPase activity and phosphorylation in submitochondrial particles involve the tight nucleotide binding sites and the (weak) ATPase site, while uncoupled ATPase activity involves only the weak site.