It has been shown that microcirculation is hypersensitized to endothelin1 (ET-1) following endotoxin (lipopolysaccharide [LPS]) treatment leading to an increased vasopressor response. This may be related in part to decreased activation of endothelial nitric oxide synthase (eNOS) by ET-1. eNOS can also be uncoupled to produce superoxide (O2). This aberrant eNOS activity could further contribute to the hyperconstriction and injury caused by ET-1 following LPS. We therefore tested whether LPS affects ROS production by vascular endothelial cells and whether and how this effect is altered by ET-1. Human umbilical vein endothelial cells (HUVEC) or human microvascular endothelial cells (HMEC) were subjected to a 6-h treatment with LPS (250 ng/mL) or LPS and sepiapterin (100 μM) followed by a 30-min treatment with 100 μM L-Iminoethyl Ornithine (L-NIO) an irreversible eNOS inhibitor and 30-min treatment with ET-1 (10 nM). Conversion of [H]L-arginine to [H]L-citrulline was used to measure eNOS activity. Superoxide dismutase (SOD) inhibitable reduction of Cytochrome-C, dihydro carboxy fluorescein (DCF), and Mitosox was used to estimate ROS. LT-SDS PAGE was used to assess the degree of monomerization of the eNOS homodimer. Stimulation of HUVECs with ET-1 significantly increased NO synthesis by 1.4-fold (P < 0.05). ET-1 stimulation of LPS-treated HUVECs failed to increase NO production. Western blot for eNOS protein showed no change in eNOS protein levels. LPS alone resulted in an insignificant increase in ROS production as measured by cytochrome C that was increased 4.6-fold by ET-1 stimulation (P < 0.05). L-NIO significantly decreased ET-1-induced ROS production (P < 0.05). Sepiapterin significantly decreased ROS production in both; unstimulated and ET-1-stimulated LPS-treated groups, but did not restore NO production. DCF experiments confirmed intracellular ROS while Mitosox suggested a non-mitochondrial source. ET-1 treatment following a chronic LPS stress significantly monomerized the eNOS homodimer that was inhibited by sepiapterin loading. The two concomitant phenomena of decreased NO production and increased ROS formation seem to be multifactorial in nature with ROS production dependent upon pterin availability.