BACKGROUND Autosomal dominant polycystic kidney disease (ADPKD) is characterized by hypertension and renal vasoconstriction. Mediators of these hemodynamic changes are not well understood, but evidence suggests that endothelial-derived mediators may participate. METHODS Baseline measurements of blood pressure, proteinuria, and urinary nitrite/nitrate excretion were performed in control and cystic male Han:SPRD rats (6 weeks of age). They were then treated with the nitric oxide (NO), nitric oxide synthase (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), or vehicle, for 6 weeks. After repeat systemic measurements, renal function was determined using inulin and para-aminohippurate (PAH) clearances. Levels of renal endothelin-1 (ET-1) and renal endothelial NOS (eNOS) proteins were determined, and immunohistochemistry localized renal eNOS and neuronal NOS (nNOS). RESULTS Administration of L-NAME aggravated systemic hypertension and renal vasoconstriction in the cystic rats, but did not affect the progression of proteinuria or cystic expansion. Cystic rats demonstrated marked increases in renal ET-1 and eNOS levels. L-NAME reduced eNOS expression in the membrane compartment, but increased eNOS in the cytosol. Localization studies indicated that renal eNOS was abundant in nonvascular compartments, but not in renal vascular and glomerular structures, whereas renal nNOS was diffusely diminished. CONCLUSIONS These alterations of endothelial-derived mediators (up-regulation of ET-1, and dysfunction of the NO system) contribute to vasoconstriction, and thereby are likely to contribute to the progressive loss of renal function in polycystic kidney disease (PKD).