The developing mammal exhibits lower renal blood flow (RBF) and higher renal vascular resistance (RVR) than its adult counterpart. The maturational pattern of renal hemodynamics involves the synchronous increase in RBF and decrease in RVR with age. In spite of considerable investigation, the mechanisms involved in the regulation of renal hemodynamics in the developing animal remain largely unexplained. Specifically, the role of the vasodilator endothelium-derived nitric oxide (EDNO) in the regulation of developing renal hemodynamics is not known. These experiments examined the intrarenal effect on the renal hemodynamics of the developing piglet and adult pig of the EDNO competitive inhibitor N-nitro-L-arginine methylester (L-NAME) at three doses (50, 5, and 3 μg/kg/min). During basal conditions, the developing piglet exhibited lower RBF and higher RVR than the adult pig. All doses of intrarenal L-NAME produced significant decreases in RBF and increases in RVR in both groups. The 3-μg/kg/min L-NAME dose did not change mean arterial pressure. The developing piglet exhibited significantly greater changes at all doses. After the 50-μg/kg/min infusion, piglet RBF decreased 45% and adult pig RBF decreased 29%; piglet RVR increased 128% and adult pig RVR increased 51%. After a 5-μg/kg/min infusion, RBF decreased 28% in the piglet and 14% in the adult pig; RVR increased 75% in the piglet compared with 27% in the adult pig. After 3 μ/kg/min L-NAME, piglet RBF decreased 29% and adult RBF decreased 9%; RVR increased 47% in the piglet versus 13% in the adult pig. The results of this study suggest that EDNO participates in the regulation of basal renal hemodynamics in the developing piglet and adult pig. Furthermore, it appears that EDNO may play a greater role in maintaining basal renal hemodynamics in the developing piglet than in the adult pig.