Under the conditions of the present study, the chief limitation to success in long-term preservation of the dog kidney (48- and 72-hour) was early thrombosis of preserved renal vessels at the site of suture anastomosis. Endothelial damage may be the initiating factor and may be modified by the use of additives. Parenchymal function was good and there was little evidence of damage on histologic studies. In spite of frequent contamination of the perfusate, sepsis did not have an important role in the failures observed. A moderate increase in perfusion pressure and the presence of tissue edema during the course of preservation were not prognostically significant.