The development of filtration rate and perfusion rate in superficial nephrons has been studied in 17- to 60-day-old rats with micropuncture and microsphere techniques. Recordings have also been made of total renal blood flow (RBF), cardiac output (CO), arterial blood pressure, and hematocrit. Sincle-nephron glomerular filtration rate (SNGFR) as well as glomerular perfusion rate (GPR) and RBF described a similar developmental pattern with a slower rise from 17 to 30 days of age and a faster rise thereafter. No plateau was observed. The quotient between SNGFR and GPR was constant from at least 22 days of age. Since the hematocrit was also constant after 22 days this implies a constant filtration fraction. On the basis of those findings it is suggested that GPR is the main determinator for the development of GFR. The fraction of cardiac output perfusing the kidneys increased continuously in 17- to 60-day-old rats, suggesting that the increase in RBF and GPR is due to intrarenal factors: renal vasodilatation and increase in renal vascular volume.