Successful 6-day kidney preservation was obtained with a combination of perfusion techniques. Canine kidneys were preserved by hypothermic continuous perfusion in a Gambro machine with albumin perfusate. On day 3 the hypothermic preservation was interrupted by a normothermic perfusion for 3 hr. This intermediate perfusion was performed in a heart-lung machine filled with heparinized blood from non-related dogs. The preserved kidneys were autotransplanted in subsequently nephrectomized dogs. Nine of 11 animals with such preserved kidneys survived in a healthy condition. In the control group with hypothermic perfusion only, one of 8 dogs survived after implantation. In this dog a high serum creatinine concentration was measured after implantation. Kidney function was assessed by determining glomerular filtration rate (GFR), effective renal plasma flow (ERPF), and filtration fraction (FF) during the intermediate blood perfusion in the experimental group and at 1 hr after implantation in both groups. A low FF (less than 0.27) indicated the presence of active functioning tubules in the intermediate blood-perfused group. In this group the GFR, ERPF, and FF values returned to normal within 10 days after implantation. These results show that it is possible to prevent irreversible preservation damage by normothermic blood perfusion. Histological observation indicated that the isolated blood perfusion had a beneficial effect, especially on the tubules of the preserved kidney.