Extract: The developmental pattern of the glomerular filtration rate (GFR), the regional glomerular density, and glomerular blood flow, as well as that of the cardiac output, blood pressure, and hematocrit has been followed in 2—79-day-old lambs. The glomerular perfusion rate (GPR), i.e., the glomerular blood flow, has been derived by determining the regional renal blood flow with microspheres and dividing it by the number of glomeruli per unit of renal tissue at that cortical depth assuming that practically all of the spheres are trapped in the glomeruli. The GFR per gram of renal tissue increased more rapidly during the first days of life, whereas the total GFR appeared to increase more linearly with age. The glomerular density fell in all cortical layers. The fall was slightly more pronounced in the inner cortex. The glomerular blood flow was recorded in three zones: the outer 10% of the cortex, which contained an aglomerular as well as a high density glomerular zone; the mid 65% of the cortex, which on gross inspection appeared fairly homogeneous; and the inner 25% of the cortex, which corresponded to the juxtamedullary zone. In all zones the GPR increased with age. The increase was most pronounced in the inner cortex in the oldest animals. The largest scatter of data was found in the outer cortex as a result of the heterogeneity of that zone. The cortical profile for the GPR did not change much with age, which indicated that in the lamb no intrarenal redistribution of blood flow of greater importance occurs during the postnatal development. In the inner and midcortex the GPR increased in proportion to the cardiac output. In the outer cortex the data indicate that GPR increased somewhat more rapidly than cardiac output. The development of the total GFR has been related to the development of the GPR in the inner and midcortex since those two zones contain more than 90% of all glomeruli. During the first period of life the GFR increased more rapidly than the GPR. This coincided with the time when the most rapid fall in hematocrit was recorded. Thus the development of the glomerular plasma flow and the GFR appeared to follow closely. Speculation: In the growing animal as well as in the adult the glomerular plasma flow appears to be the main determinator of the glomerular filtration rate (GFR). Inasmuch as the arterial blood pressure in infants is close to the level at which the renal blood flow is autoregulated in the mature kidney, the GFR might be more susceptible to fluctuations in infants.