Fifty dog kidneys had hypothermic perfusion for 12 to 72 hours and then were transplanted. Prior to perfusion the kidneys were subjected to 15, 30, 45, and 60 minutes of warm ischemia. Fifteen minutes of warm ischemia was well tolerated (successful 72 hour preservation), but after 30 minutes successful preservation could be achieved for 24 hours only. Enzyme release and lactate formation were related to the prolongation of warm ischemia, and the highly significant concentration differences of these substances, between well functioning kidneys and those with small or no function, indicated the viability of the organ.