Tumor necrosis factor- (TNF-) is elevated in the plasma of preeclamptic women and may have a role in pregnancy-induced hypertension. However, whether the hemodynamic effects of TNF- reflect the direct effects on vascular reactivity is unclear. We tested the hypothesis that TNF- impairs endothelium-dependent relaxation and enhances vascular contraction in systemic vessels of pregnant rats. We measured isometric contraction in aortic strips isolated from virgin and pregnant Sprague-Dawley rats (nontreated vs. treated for 2 h with 10-1,000 pg/ml TNF-). In endothelium-intact vascular strips, TNF- caused greater enhancement of phenylephrine (Phe) contraction in pregnant than virgin rats. TNF- caused significant inhibition of ACh- and bradykinin-induced vascular relaxation and nitrite/nitrate production that were more prominent in pregnant than virgin rats. N^G-nitro-L-arginine methyl ester [L-NAME, 100 µM, an inhibitor of nitric oxide (NO) synthase] or 1H-[1,2,4]oxadiazolo[4,3]-quinoxalin-1-one (ODQ, 1 µM, an inhibitor of cGMP production in smooth muscle) inhibited ACh relaxation and enhanced Phe contraction in nontreated but to a lesser extent in TNF--treated vessels, particularly those of pregnant rats. Endothelium removal enhanced Phe contraction in nontreated but not TNF--treated vessels, especially those of pregnant rats. Relaxation of Phe contraction with the NO donor sodium nitroprusside was not different between nontreated and TNF--treated vessels. Thus TNF- enhances vascular contraction and inhibits endothelium-dependent NO-cGMP-mediated vascular relaxation in systemic vessels, particularly those of pregnant rats. The results support a direct role for TNF- as a possible mediator of increased vascular resistance associated with pregnancy-induced hypertension.