T he intrarenal distribution of radionuclide microspheres injected into the thoracic aorta was used to examine glomerular blood flow distribution (GBFD) in 26 healthy, unanesthetized puppies, ranging in age from 5 h to 42 days, and in 5 adult dogs. For analysis, the cortex was divided into four equally thick zones designated zone I (subcapsular) to zone IV (juxtamedullary).During the first 36 h of life, the highest flow rate was in zone II, which received 35.5±2.0%/g, compared with 26.8±1.4% to zone I, 23.7±1.4% to zone III, and 13.4±1.4% to zone IV. At age 6 wk, zone I had the highest rate of perfusion (48.6±2.1%, compared with 28.8±1.4% in zone II, 15.8±0.8%, in zone III, and 6.8±0.6% in zone IV). The 6-wk old animals resembled the adult animals, except for relatively greater perfusion per gram of zone I in the former group. Changes in relative GBFD did not correlate with those in arterial pressure or peripheral hematocrit.The distribution of glomeruli among the four zones of the cortex followed its own pattern of development. At birth and at 6 wk, the greatest density of glomeruli was in zone I (50.6±5.4 and 42.7±3.9%/g respectively, as compared with 24.1±2.9% in adults); in adults zone II contained the greatest density (39.1±1.6%).At birth the relative perfusion of glomeruli in zone I was only one-fifth that of glomeruli in zone IV, with intermediate values in zones II and III. By 6 wk of age, increased perfusion of the outer cortical glomeruli resulted in rates of flow in the four zones that did not differ significantly from each other. Relative perfusion in zone I continued to increase, so that in the adult animals perfusion in that zone was significantly greater than in the three deeper zones.These data demonstrate the marked hemodynamic changes that take place within the kidney during the first few weeks of life. The relatively greater blood flow of the most deeply situated nephrons in the early postnatal period suggests ascendancy of this population of nephrons and may have important functional implications.