The present experiments were carried out to investigate the role of endogenously produced NO in modulating renal function during postnatal maturation under physiological conditions. In conscious, chronically instrumented lambs aged ~1 (n = 8) and ~6 wk (n = 8) of postnatal life, various parameters of glomerular and tubular function were measured for 1 h before and 1 h after intravenous injection of 20 mg/kg of N^G-nitro-L-arginine methyl ester (L-NAME; experiment 1) or its inactive isomer D-NAME (experiment 2). After administration of L-NAME to 1-wk-old lambs, glomerular filtration rate (GFR) and filtration factor (FF) decreased by ~50% at 20 min, remaining decreased at 60 min. In 6-wk-old lambs, GFR and FF remained constant after L-NAME. Proximal fractional Na⁺ reabsorption decreased after L-NAME administration to lambs aged 6 wk, resulting in a prompt natriuresis; this was sustained for 60 min. There were no effects of L-NAME on proximal fractional Na⁺ reabsorption in 1-wk-old lambs. In 6-wk-old lambs, urinary flow rate increased by ~500%, free water clearance increased by ~50%, and urinary osmolality decreased by ~60% after L-NAME administration; no effects on these variables were measured in 1-wk-old lambs. The diuresis after L-NAME administration to 6-wk-old lambs was unaccompanied by any changes in plasma levels of arginine vasopressin. There were no effects of D-NAME on any of the measured variables. We conclude that endogenously produced nitric oxide modulates glomerular and tubular function in an age-dependent manner.