SummaryRenal uric acid excretion was examined in 35 mongrel puppies at 1,30,60 and 90 days of age and in seven mature mongrel dogs. Uric acid concentrations in plasma and urine were determined using a uricase fluorometric method. Clearance of inulin increased with postnatal age; however, fractional excretion of uric acid (FEUA) decreased from 83% at birth to 51% at 90 days of age (r= −0.675,P< 0.001). Filtered uric acid and net reabsorption of uric acid increased with postnatal development. The decline in FEUAwith postnatal age was unrelated to binding of uric acid to plasma proteins or to urine flow rate. A direct correlation was observed between clearance of uric acid (CUA) and clearance of sodium (CNa) during early development (r= 0.48,P< 0.001). These data indicate that postnatal maturational patterns of renal urate excretion in mongrel puppies are similar to human newborns.SpeculationThe high FEUAobserved during early postnatal development in the puppy represents decreased tubular reabsorption and/or increased renal tubular secretion of uric acid. In early development, tubular reabsorption of uric acid may be diminished as a result of the high extracellular fluid volume (ECFV). The persistence of high FEUAin puppies at 2 and 3 months of age may be the result of physiologic factors which are different than those in the first day of life. The mongrel puppy appears to be an appropriate animal model for further investigations of renal uric acid transport during human development