SummaryThe ontogeny of single nephron flitration distribution was studied in 29 canine puppies two hr to 42 days old using the [14C] ferrocyanide method as modified by Coehlo (9). The ratio of14Counts in outer cortical nephrons (OC) to counts in juxtamedullary nephrons (JM) were plotted against age for each puppy. This ratio (OC:JM) provided an index for comparing the single nephron mlomerular filtration between regions. A four-fold increase in OC:JM occurred from two days to four wk. The youngest puppy, two or old, showed a ratio of zero because no counts were detected in OC at that age.The most rapid increase occurred in the first 8 days of life. By the fourth wk, the mean OC:JM ratio was 0.72 ± 0.06, approaching values reported by other laboratories for adult dogs. In contrast to age-related changes in OC:JM ratio, the ratio of middle cortical to JM counts showed no age-related increase. Cortical blood flow distribution was determined by the microsphere method. The ratio of blood flow to outer and inner cortex increased from 0.43 at one-half a day to a mean of 2.93 after one wk. The ratio of OC:JM single nephron nitration to OC/inner cortical nephron renal blood low was 0.36 from 3 days to 26 days (n= 12) and did not change significantly with age, indicating that changes in zonal filtration matched the alterations in cortical blood flow distribution.SpeculationThese studies suggest that although systemic arterial pressure changes probably play a role in the increasing single nephron filtration in the outer cortex of the maturing puppy, it is likely that plasma flow is a governing factor.