There are 2 competing methods for cooling the kidney in situ during surgical ischemia: from without by applying ice to the renal surface and from within by perfusing the renal artery. The latter procedure is said to be superior in protecting renal function. Herein the protective effect on renal function of both methods are compared. Pigs of 15--25 kg weight underwent nephrectomy on one side. The remaining kidney was subjected to cold ischemia during 90 minutes while perfusion- or surface cooling was performed. For perfusion cooling the aorta was punctured and the catheter introduced into the renal artery. The perfusing liquid consisted of a physiologic electrolyt solution (Ringer-Lactate) with heparin kept at a temperature of 3--5 degrees C. The initial perfusion lasted 10 minutes and resulted in a median renal core temperature of 23 degrees C. Then the kidney was put on a cooling pad and every 15 minutes again perfused for one minute. For surface cooling sterile melting ice made of glucose solution 5% was applied directly to the kidney. The renal core temperature could be kept at 15--20 degrees C. The two methods of hypothermia were judged by comparing the serum creatinine levels and the I131-hippuran clearances one month after surgery. There was no difference whatever as analysed by the t-test. Hypothermia by applying ice to the renal surface therefore proved to be equivalent to hypothermia by perfusion. Moreover it is much simpler.