SUMMARYTissue levels of adonosine triphosphate (ATP) have been measured in rat kidneys following periods of warm ischaemia: (1) immediately after the ischaemic period and (2) after the kidney had been reperfused with blood for 10 min. ATP levels at the end of the period of ischaemia are similar for ischaemic periods of 10 to 60 min and give no indication of the kidneys subsequent functional ability. The amount of ATP regenerated in 10 min of reperfusion correlates both with the duration of the period of ischaemia and with the subsequent functional ability of the kidney.