1. The maturation of glomerular filtration rate was studied by comparison of thirty-six new-born mongrel dogs aged 1-35 days with six adult dogs.2. Under mannitol diuresis, glomerular filtration rate (GFR) rose from 0.16 ml. min(-1).g kidney(-1) at 1 day of age to 0.34 ml. min(-1).g kidney(-1) at 1 month of age. Adult GFR averaged 0.68 ml. min(-1).g(-1). There was good correlation of GFR with arterial blood pressure (r = 0.76, P < 0.001). Part of the statistical correlation of GFR with blood pressure was found to be independent of the relationship between blood pressure and age.3. Acute increases or decreases in blood pressure resulted in parallel changes in GFR in the puppies. There was no change of GFR with change of blood pressure in adult dogs. Carotid artery clamping, independent of blood pressure changes, produced increased renal vascular resistance and decreased GFR in the pups.4. Renal plasma flow (RPF) increased from 0.70 ml. min(-1).g(-1) at 1 day of age to 1.80 ml. min(-1).g(-1) at 1 month and showed good correlation with blood pressure (r = 0.67, P < 0.001). Filtration fraction (GFR/RPF) and renal vascular resistance did not vary with age in the pup and were the same as those for the adult.5. These results support the hypothesis that maturation of GFR and RPF are closely related to maturation of arterial blood pressure in the mongrel dog. The factors other than blood pressure which also affect renal maturation in the dog still need to be more clearly defined.