Background and Objective: Although vascular endothelial cell function (i.e., the release of endothelium-derived nitric oxide) decreases and plasma tumor necrosis factor (TNF) increases during sepsis, it is not known whether the elevated TNF is responsible for the depression of endothelial cell function under such conditions. The aim of this study, therefore, was to determine if inhibition of TNF biologic activity by polyethylene glyeol dimerized conjugate of the recombinant human form of the p55 soluble TNF receptor (PEG-(rsTNF-R1)2) maintains endothelial function during sepsis. Design, Materials, and Methods: Rats were subjected to sepsis by cecal ligation and puncture (CLP). Immediately before the onset of sepsis, 600 μg/rat PEG-(rsTNF-R1)2 or an equal volume of saline was infused intravenously. At 10 hours after CLP (i.e., hyperdynamic sepsis), the thoracic aorta was isolated, cut into rings, and placed in organ chambers. Dose responses for an endothelium-dependent vasodilator, acetylcholine (ACh), and an endothelium-independent vasodilator, nitroglycerine (NTG), were determined. Endothelial cell structure was examined by transmission electron microscopy. Results: Endothelium-dependent vascular relaxation was depressed at 10 hours after the onset of sepsis. Administration of PEG-(rsTNF-R1)2 before CLP, however, maintained ACh-induced relaxation. In contrast, no significant difference in NTG-induced relaxation was seen, irrespective of administration of PEG-(rsTNF-R1)2. Furthermore, the deterioration in endothelial structure during sepsis was prevented by PEG-(rsTNF-R1)2 pretreatment. Conclusion: Since administration of PEG-(rsTNF-R2)2 maintains vascular endothelial cell structure and function, it can be concluded that TNF plays a pivotal role in producing endothelial dysfunction during sepsis. Thus, pharmacologie agents that inhibit TNF biologic activity and/or its production may be useful for protecting endothelial cells during sepsis.