Work in our laboratory has focused on the role of nitric oxide synthase (NOS) in the regulation of renal medullary function. Biochemical studies demonstrated that the renal medulla is enriched in immunoreactive NOS protein and NOS enzymatic activity when compared with the renal cortex. Further experiments showed large amounts of NOS activity in the inner medullary collecting ducts, while moderate NOS activity was found in glomeruli and vasa recta and minimal NOS activity was detected in other nephron segments examined. In subsequent functional studies, selective renal medullary infusion of NOS stimulators (bradykinin or acetylcholine) or inhibitors (L-NAME) preferentially altered medullary blood flow. The alterations in medullary flow were associated with parallel changes in sodium and water excretion. Similar to the effects observed in anaesthetized rats, chronic infusion of L-NAME directly into the renal medullary interstitial space of conscious, uninephrectomized SD rats selectively decreased renal medullary blood flow throughout a 5-day L-NAME infusion period. The decrease in medullary blood flow was associated with retention of sodium and the development of hypertension, and the effects were reversible. In contrast to the effects of chronic NOS inhibition, renal medullary infusion of NOS substrate L-arginine prevented the development of sodium-sensitive hypertension in the Dahl salt-sensitive rat placed on a high sodium diet. The data reviewed in this paper indicate that NOS isoforms expressed in the renal medulla have a potent influence on renal medullary tubular and vascular function with consequential effects on fluid and electrolyte homeostasis and arterial blood pressure.