To retrieve cadaver kidneys for human homotransplantation, organ viability must be maintained in vivo during the preterminal period when renal perfusion is low, and in vitro during the period while the recipient is being prepared. In the laboratory, experiments were designed to simulate the clinical situation in which canine cadaver kidneys are used. It was found that general profound hypothermia before death inhibits to a significant degree the untoward effects of renal ischemia by virtue of its suppressive action on metabolic activity. General body hypothermia to 28 C (82.4 F) applied three hours before death permits removal of viable, transplantable kidneys up to two hours postmortem. No treatment at all requires immediate removal of the kidney to retain its viability. This study suggests that for successful use of cadaver kidneys in human homotransplantation, both in vivo and in vitro renal protection from ischemia are of paramount importance for ultimate success.