Recently we have had an opportunity to observe histologic changes in dog kidneys during renal autoreplantation studies. While attempting to increase the period of ischemia tolerated by the dog kidney, biopsies have been taken of kidneys during various stages of extracorporeal hypothermia. The present study was designed to provide a serial histologic record of the changes found in a hypothermic kidney which resumed normal, life-sustaining, function after a prolonged interval outside the donor animal's body. By restricting our investigation to autoreplantations (kidney regrafted into original site in donor with anastomoses to original vascular supply, and end-to-end ureteral anastomosis) the well-recognized pathologic changes of the homograft rejection phenomenon were avoided. Changes observed in renal tubular cells could be more accurately attributed to hypothermia, ischemia, thrombosis, or infection. The introduction of general body hypothermia as an adjunct to cardiac surgery has again directed attention to the possible benefits of local cooling in