We used high-resolution phosphate 31 (31P)–nuclear magnetic resonance spectroscopy to study the effects of ischemia and reperfusion on intracellular adenosine triphosphate (ATP) and pH changes in isolated perfused rat kidneys. With renal Ischemia, ATP levels fell rapidly and the inorganic phosphate (Pi) peak shifted, indicating acidosis. On reperfusion after 45 minutes of warm ischemia, there was a 56% rise in tissue ATP levels within ten minutes that then slowly declined; by 75 minutes the levels were only 33% of normal. Perfusate flow decreased from 21.2± 0.9 mL/min (mean ± SE) to 16.5±1.1 mL/min and the Pi peak did not shift during reperfusion. When 0.3mM ATP complexed to magnesium chloride (ATP-MgCl2) was added to the perfusate after ischemia, renal ATP levels increased to 69% of normal within ten minutes of reperfusion and by 75 minutes they were normal. Perfusate flow was also normal during reperfusion. The Pi peak shifted back to the normal frequency, indicating correction of the intracellular acidosis. Thus, intracellular acidosis, ATP depletion, and decreased flow during reperfusion injury were rapidly reversed and sustained by the postischemic administration of ATP-MgCl2. © 1985, American Medical Association. All rights reserved.