Sprague-Dawley rats were nephrectomized unilaterally, or sham-operated at 5, 12, and 40 days of age. The kidney weight, number of glomeruli, GFR, and single nephron GFR (SNGFR) were determined in about 60 days of age. In both sham-operated and nephrectomized rats operated on at 5 and 12 days of age, the kidney was studied with light microscope for 1 to 14 days. Unilateral nephrectomy performed just before or just after completion of nephrogenesis or in adulthood did not result in formation of new nephrons. Renal structural development followed the same pattern in nephrectomized and sham-operated rats. A compensatory increase in renal size and GRF was inversely related to the age at which the nephrectomy was carried out but was more pronounced if nephrectomy had been performed just before rather than just after completion of nephrogenesis. In all nephrectomized rats, the compensatory increase in filtration rate appears to be the same in nephrons at all cortical levels judging by comparison of SNGRR and GFR. Conclusion. Compensatory renal growth does not involve the formation of new nephrons in the postnatal kidney of the rat. The potentiation of compensatory changes after nephrectomy at an early age is largely dependent on the development stage of the kidney at the time of nephrectomy.