Inflammation is a condition that underscores many cardiovascular pathologies including endothelial dysfunction, but no link is yet established between the vascular pathology of the metabolic syndrome with a particular inflammatory cytokine. We hypothesized that impairments in coronary endothelial function in the obese condition the prediabetic metabolic syndrome is caused by TNF-α overexpression. To test this, we measured endothelium-dependent (acetylcholine) and -independent vasodilation (sodium nitroprusside) of isolated, pressurized coronary small arteries from lean control and Zucker obese fatty (ZOF, a model of prediabetic metabolic syndrome) rats. In ZOF rats, dilation to ACh was blunted compared with lean rats, but sodium nitroprusside–induced dilation was comparable. Superoxide (O₂·⁻) generation was elevated in vessels from ZOF rats compared with lean rats, and administration of the O₂·⁻ scavenger TEMPOL, NAD(P)H oxidase inhibitor (apocynin), or anti–TNF-α restored endothelium-dependent dilation in the ZOF rats. Real-time PCR and Western blotting revealed that mRNA and protein of TNF-α were higher in ZOF rats than that in lean rats, whereas eNOS protein levels were reduced in the ZOF versus lean rats. Immunostaining showed that TNF-α in ZOF rat heart is localized in endothelial cells and vascular smooth muscle cells. Expression of NAD(P)H subunits p22 and p40-phox were elevated in ZOF compared with lean animals. Administration of TNF-α more than 3 days also induced expression of these NAD(P)H subunits and abrogated endothelium-dependent dilation. In conclusion, the results demonstrate the endothelial dysfunction occurring in the metabolic syndrome is the result of effects of the inflammatory cytokine TNF-α and subsequent production of O₂·⁻.