Although depressed endothelium-dependent relaxation occurs during early sepsis, the precise mechanism responsible for this remains unknown. Because the elevated levels of plasma tumor necrosis factor (TNF) play a major role in the pathophysiology of sepsis, we investigated whether TNF-α administration alters endothelium-dependent relaxation. To study this, recombinant TNF-α (1.2 x 107 U/mg) was infused intravenously (0.25 mg/kg body wt) for 0.5 h in normal rats, and mean arterial pressure was monitored. At 1 h after the completion of TNF-α or vehicle infusion, the aorta and a pulmonary artery were isolated, cut into 2.5-mm rings, and placed in organ chambers. Norepinephrine (2 x 10-7 M) was applied to achieve near-maximal contraction, and dose responses for an endothelium-dependent vasodilator, acetylcholine, and an endothelium-independent vasodilator, nitroglycerine, were determined. In additional studies, aortic rings from normal animals were incubated with TNF-α for 2 h in vitro, and vascular reactivity was determined. The results indicate that TNF-α administration significantly reduced acetylcholine-induced vascular relaxation both in vivo and in vitro. Such a reduction was sustained at least 80 min after the completion of 2-h incubation with TNF-α. In contrast, TNF did not alter nitroglycerine- induced vascular relaxation. Thus TNF-α depresses endothelium-dependent relaxation in vitro as well as in vivo. Because TNF-α infusion increases plasma TNF levels without decreasing mean arterial pressure, the depressed endothelium-dependent relaxation observed during early sepsis may be due to the elevated circulating levels of TNF.