The rate of exchange between inorganic phosphate and ATP was measured in isolated perfused rat livers in the direction of ATP synthesis using 31P NMR spectroscopy and the saturation-transfer technique. Measurement of ATP hydrolysis was not observable, even after treatment of rats with 100 micrograms T3/day per 100 g body wt. When the perfused livers were treated with iodoacetate in order to inhibit glycolysis, NMR measurable exchange between ATP and Pi was eliminated. It is concluded that the inorganic phosphate----ATP conversion detected by saturation transfer is catalyzed by enzymes of the glycolytic pathway and that the mitochondrial ATPase rate is too slow to contribute to the observed effect.