Abstract Background Preeclampsia is associated with increases in plasma levels of tumor necrosis factor–α (TNF-α), a cytokine known to contribute to endothelial dysfunction. We recently reported that a twofold elevation in plasma TNF-α produces significant reductions in renal function and hypertension in pregnant rats. The purpose of this study was to determine the role of the nitric oxide (NO) system in TNF-α–induced hypertension in pregnant rats. Methods Tumor necrosis factor–α (50 ng/day) was chronically infused starting at day 14 of gestation. Mean arterial pressure, 24-h urinary nitrite/nitrate excretion, and renal nitric oxide synthase (NOS) protein expression by Western blot analysis was measured at day 19 of gestation. Results A twofold increase in plasma TNF-α levels in pregnant rats resulted in a significant increase in arterial pressure (97 ± 3.6 v 116 ± 2.1 mm Hg, pregnant versus TNF-α pregnant, respectively, P < .05), but no significant change in urinary nitrite/nitrate excretion (22.0 ± 1.9 v 20.8 ± 2.5 μmol/24 h, pregnant versus TNF-α pregnant, respectively), a measure of whole body NO production. As abnormalities in renal production of NO would not be reflected in the measure of whole body NO production, changes in renal NOS protein levels were determined. The protein expression of both neuronal (nNOS) and inducible (iNOS) nitric oxide synthase were significantly decreased in the medulla of TNF-α pregnant rats (nNOS: 10.6 ± 0.7 v 8.2 ± 0.8 densitometric units, P < .05; and iNOS: 19.2 ± 0.9 v 15.4 ± 0.8 densitometric units, P < .05, pregnant versus TNF-α pregnant, respectively). Conclusion: The hypertension associated with a chronic twofold increase in TNF-α in pregnant rats is associated with significant decreases in renal nNOS and iNOS protein production.