at this temperature kidney cortical cells ATPase is still active. These experiments were performed on aortic endothelium, but if the vascular endothelium of the kidney has the same properties, then vascular injury may be the limiting factor in prolonged kidney storage. Willis has shown how hibernators derive some of their ability to survive low temperatures by having a Na+K+-ATPase which is able to transport sodium and potassium actively across their cell membranes, at 5 °C, about six times faster than in non-hibernators; this transport rate is sufficient to prevent cell swelling. The rate of cooling of a tissue may also be significant in the production of injury to enzyme systems. Francavilla showed that when liver slices were rapidly cooled (immediate cooling to 12 °C in 6 minutes) anaerobic glycolysis, as measured on rewarming to 37 °C, was inhibited by about 67% of the activity that was demonstrated in slices that had been subjected to delayed cooling. However, dog kidney slices were less severely affected by the rapid cooling than were the liver slices. Anoxia All cells require ATP as an energy source for their metabolic activity. The kidney is damaged by anoxia when kidney cortical cells are unable to generate sufficient ATP under anaerobic conditions to meet the needs of the cells. When excising a kidney some anoxia is inevitable in the interval between dividing the renal artery and cooling the kidney. It has been shown by Bergstrom that 50% of a dog's kidney's cortical cells ATP content is lost within 1 minute of clamping the renal artery, and similar results were found by Warnick in whole mice kidneys, with a fall in cellular ATP by 50% after about 30 seconds of warm anoxia. Warnick and Bergstrom also showed that cooling the kidney immediately after removal markedly reduced any further ATP loss. When these non warm-injured kidneys were perfused with oxygenated hypothermic plasma, ATP levels were reduced by 50% after 24-hour storage and, after 48 hours, mean tissue ATP levels were a little higher than this indicating that synthesis of ATP had occurred. Pegg has shown that rabbit kidneys can resynthesize ATP after a period of perfusion storage following warm injury, but no resynthesis occurred in non warm-injured kidneys. Warm anoxia can also occur during reimplantation of the kidney after storage. Lannon showed, by measurements of succinate metabolism, how the kidney was more sensitive to a period of warm hypoxia