Rat liver mitochondrial adenylate kinease [ATP: AMP phosphotransferase, EC 2. 7. 4. 3] was observed to “release from” and “rebind to” mitochondria. The following conditions influence this observed adenylate kinase flux: 1. Phosphate caused a more rapid release and an alkaline pH allowed a greater extent of adenylate kinase release from mitochondria. 2. None of the nucleotides, which were tested, including ATP, ADP, and AMP, prevented the phosphate induced release. 3. All divalent cations, which were tested, completely inhibited the phosphate induced release. 4. Uncouplers of oxidative phosphorylation partially prevented the phosphate induced release. Atractyloside had no effect. 5. The swelling of the mitochondria paralleled release of adenylate kinase from the mitochondria. 6. Released adenylate kinase activity will “rebind” to mitochondria which are partially swollen, but will not “rebind” to completely swollen mitochondria.