One hour of warm ischemia produced no detectable injury in canine kidneys, provided the dog was previously heparinized. Two hours of warm ischemia produced severe renal injury resulting in the death of three dogs out of four. Renal ischemic tolerance could be extended to two hours by means of a simple peraortic renal arterial flush with either cold 0.9% saline or cold hyperosmolar saline. The ischemic interval could, however, be extended to three hours when the kidney was maintained at room temperature (22 C) after a simple cold saline flush. Cold hyperosmolar saline conferred no greater benefit than simple hypothermia.