Nitric oxide (NO) may play a more important vasoactive role in the developing kidney than the adult, perhaps related to a different developmental intrarenal distribution of the NO synthesizing enzyme, nitric oxide synthase(NOS). These experiments determined the intrarenal histochemical localization of NOS in developing piglets ages 2 days, 1 week, 3 weeks and adult pigs. Kidneys were fixed by in-vivo perfusion, and placed in increasing sucrose concentrations for 72 hr. 15 micron cryostat sections were stained with the NADPH-diaphorase technique, which demonstrates the catalytic activity of NOS by the enzymatic reduction of nitro blue tetrazolium, NBT, in the presence of NADPH. Localization of NOS in the immature kidney differed from the adult in three major patterns: 1. NOS was present in the macula densa at all stages of nephron development, identifying the morphologic formation of this segment. 2. NOS localized to glomerular resistance arterioles in the developing kidney, most intensely in the 1 and 3 week old piglets. 3. All developing age groups intensely demonstrated NOS in segments of the thick ascending limb. In the adult, NOS was confined primarily to the macula densa, with faint staining of resistance arterioles and thick ascending limb. Summary: Intrarenal histochemical localization of NOS reveals distinct developmental localization patterns for the macula densa, glomerular resistance arterioles and tubular segments in the immature kidney that differ from the adult.Conclusion: The identification of NOS in these locations supports the role of NO as an important participant in developing renal function.