We report a study of the effects of perfusate composition on the 5'-adenine nucleotides of rabbit kidneys perfused continuously at 10 C for 24 or 48 hr. It was found that when glucose was the sole energy source and the pO2 was approximately 150 mm Hg, the total high-energy phosphate content was reduced to one-half and total adenine nucleotides to one-third after 48-hr perfusion of freshly isolated kidneys. At this pO2 the supplementation of glucose of additional energy substrates had no effect on adenine nucleotide reserves, but the inclusion of hypoxanthine did have a positive effect, although not sufficient to restore normal levels. Increasing the pO2 to approximately 650 mm Hg however, in combination with the provision of energy substrates and hypoxanthine, produced adenine nucleotide levels very close to normal after 48 hr of perfusion. Study of metabolites in the perfusate suggested that significant glycolysis occurred when the pO2 was 150 mm Hg: it was stimulated by caprylate and inhibited by raising the pO2 to 650 mm Hg. Kidneys exposed to 60 min of warm ischemia before perfusion had severely depleted adenine nucleotide levels (less than one-fifth of normal) but after 48-hr perfusion with hypoxanthine, energy substrates, and a high pO2, the total adenine nucleotides were restored to normal and only the ATP to ADP ratio was significantly less than that observed in kidneys perfused for 48 hr without prior warm ischemia. Study of metabolites in the perfusate suggested that a burst of aerobic respiration occurred after a lag period of 20 hr in these kidneys. It is clear that a remarkable recovery of adenine nucleotides can occur during perfusion, under appropriate conditions, of kidneys damaged by warm ischemia.