The rate and efficiency of energy transport were examined in a system containing isolated rabbit heart mitochondria, hexokinase, adenylate kinase and low concentrations of adenine nucleotides. Oxygen consumption by mitochondria and glucose-6-phosphate synthesis by hexokinase were registered. It was found that when adenylate kinase is active both in mitochondria and in the environmental solution, the rate and efficiency (glucose-6-phosphate/O ratio) of glucose-6-phosphate formation considerably increase. The effects of adenylate kinase activity are fully abolished by diadenosine pentaphosphate, an inhibitor of adenylate kinase.