We have previously shown that nitric oxide (NO) is a more important intrarenal vasodilator in the developing animal compared with the adult. The interaction between NO and the renin angiotensin system in the developing kidney is not known. The purpose of this study was to determine the role of NO and angiotensin II in the regulation of developing renal function. We examined the effects of the inhibition of intrarenal NO synthesis with N-nitro-L-arginine methyl ester (L-NAME), 3 micro g/kg/min, intrarenally, administered after intrarenal infusion of either saline or an angiotensin II AT1 receptor antagonist [ATX (A-81988), 0.4 micro g/kg/min] in piglets, age 3 wk, and adult pigs. The developing piglet demonstrated significantly greater renal responses to L-NAME alone. Intrarenal NO synthesis inhibition after saline preinfusion decreased renal blood flow (RBF) in the piglet 29% compared with the adult pig 9%, but only in the piglet decreased GFR 31%, and increased plasma renin activity 57%. Intrarenal infusion of ATX significantly increased RBF in the piglet, 23%, although not altering RBF in the adult. The renal responses to L-NAME were significantly attenuated by ATX preinfusion in both age groups. After ATX pretreatment, L-NAME in piglets decreased RBF 14%, and abolished the change in GFR, whereas in adult pigs decreased RBF only 5%. CONCLUSIONS 1) angiotensin II may be a more important vasoconstrictor in the developing kidney and 2) NO is a more important regulator of renal function in the developing kidney through modulation of the renin angiotensin system.