We previously found that myocardial ischemia/reperfusion (I/R) initiates expression of tumor necrosis factor-α (TNF) leading to coronary endothelial dysfunction. However, it is not clear whether there is a direct relationship between levels of TNF expression and endothelial dysfunction in reperfusion injury. We studied levels of TNF expression by using different transgenic animals expressing varying amounts of TNF in I/R. We crossed TNF overexpression (TNF^++/++) with TNF knockout (TNF^−/−) mice; thus we have a heterozygote population of mice with the expression of TNF “in between” the TNF^−/− and TNF^++/++ mice. Mouse hearts were subjected to 30 min of global ischemia followed by 90 min of reperfusion and their vasoactivity before and after I/R was examined in wild type (WT), TNF^−/−, TNF^++/++ and TNF heterozygote (TNF^−/++, cross between TNF^−/− and TNF^++/++) mice. In heterozygote TNF^−/++ mice with intermediate cardiac-specific expression of TNF, acetylcholine-induced or flow-induced endothelial-dependent vasodilation following I/R was between TNF^++/++ and TNF^−/− following I/R. Neutralizing antibodies to TNF administered immediately before the onset of reperfusion-preserved endothelial-dependent dilation following I/R in WT, TNF^−/++ and TNF^++/++ mice. In WT, TNF^−/++ and TNF^++/++ mice, I/R-induced endothelial dysfunction was progressively lessened by administration of free-radical scavenger TEMPOL immediately before initiating reperfusion. During I/R, production of superoxide (O₂ ^·−) was greatest in TNF^++/++ mice as compared to WT, TNF^−/++ and TNF^−/− mice. Following I/R, arginase mRNA expression was elevated in the WT, substantially elevated in the TNF^−/++ and TNF^++/++ mice and not affected in the TNF^−/− mice. These results suggest that the level of TNF expression determines arginase expression in endothelial cells during myocardial I/R, which is one of the mechanisms by which TNF compromises coronary endothelial function in reperfusion injury.