The renal clearance test was carried out on 11 normal and nephrectomized rabbits for the purpose of the investigation of the renal insufficiency, and the results were as followed. 1. On the unilateral nephrectomy, acute renal insufficiency could be found generally, but mostly it recovered quickly to the normal level in a week. 2. On the partial nephrectomy with single kidney, sever renal insufficiency occurred immediately for these three days after the operation, and it recovered nearly to the normal level after 4 weeks. 3. Increasing of the amount of the resection, more severer renal insufficiency could be found, and particulary, glomerular filtration rate was obstructed and moreover, the recovery of the renal function has been put off and the chronic renal insufficiency could be found for a long time. 4. The amount of the renal parenchyma indispensable for livin was 1/4 of the total kidney. Every resection which has been carried out within this limit showed a temporary acute renal insufficiency. 5. The renal blood flow recovered, in general, quickly and, on the contrary, the glomrular filtration rate returned slowly to the normal level after the resection.