Acute, ischemic renal failure was induced in rats by clamping of the renal vessels for 1 h. Following the termination of the ischemic period 5 g/kg of dimethyl sulfoxide (DMSO) were administered intravenously as a 20% solution in saline. Control animals received normal saline. There were no deaths among the DMSO-treated animals. Urine flow began within 15 min of DMSO infusion. 24 h after the experiment the mean blood urea was 73 ±(SEM) 14 mg/100 ml (n = 29). All the control rats died during the week following the experiment. The mean blood urea at 24 h was 276 ± 18 mg/100 ml (n = 20). In 10 additional animals perfusion of the kidney with DMSO prior to the closure of renal vessels protected the organ from ischemic damage. These experiments have a bearing on the therapeutic approach to ischemic renal failure in man, and on preservation methods of donor kidneys for transplantation.