The renal blood supply of 144 rats was occluded for specific periods of time between 120 and 270 minutes. In half the animals the renal capsule was removed. Function was assessed by the ability of the damaged kidney to maintain life after removal of the opposite organ three weeks later. Removal of the renal capsule was found to reduce life-span by two months, which was significant at the 5% level. It is recommended that capsulotomy should be abandoned in human renal transplantation until evidence is obtained of its benefit.