Unilateral nephrectomy initiates a growth response in the contralateral kidney to compensate for the loss of a renal mass. The events which regulate this process are not very clear. We used a newly developed method, 99mTc-DTPA gamma-camera renography, to evaluate the individual kidney function for detection of the change of it. This method is simple, reliable and requires neither blood nor urine samples. Two groups of rabbits were used in the study. Those of 1.5-2 months of age were taken as the young aged group and those of 2 years of age as the old aged group. Kidney weight, glomerular filtration rate (GFR), and effective renal plasma flow (ERPF) were detected both before and 3 weeks after unilateral nephrectomy. After unilateral nephrectomy in both young and old age groups, the kidney weight increased by 119 and 38%, the GFR by 71 and 44%, and the ERPF by 116 and 35%. We conclude that nephrectomy at a young age where there is fast growth would cause the remaining kidney a compensatory hypertrophy which is much better than that in nephrectomy at an old age. The difference of glomerular filtration rate was less apparent than that of the kidney weight and renal plasma flow in both groups of rabbits.