Many diseases associated with sympathoexcitation also exhibit elevated reactive oxygen species (ROS). A recent animal study indicated that exogenous administration of the sympathetic neurotransmitter, norepinephrine (NE), increased systemic ROS via circulating leukocytes. The mechanisms contributing to this effect of NE and whether these findings can be translated to humans is unknown. Thus, we tested the hypothesis that NE increases superoxide production in human peripheral blood mononuclear cells (PBMCs) via NADPH oxidase. Primary human PBMCs were freshly isolated from healthy young men and placed in culture. Following NE (50pg/ml, 50ng/ml and 50µg/ml concentrations) or control treatments, NADPH oxidase mRNA expression (gp91^phox, p22^phox and p67^phox) was assessed using real time RT-PCR and intracellular superoxide production was measured using dihydroethidium fluorescence. PBMCs were also treated with selective adrenergic agonists-antagonists to determine the receptor population involved. In addition, CD14⁺ monocyte-endothelial cell adhesion was determined using a fluorescent-based assay. NE significantly increased NADPH oxidase gene expression and intracellular superoxide production in a time dependent manner (superoxide: 0.9±0.2 fold 6 hours vs. 3.0±0.3 fold 36 hours (NE, 50μg/ml); P < 0.05). The sustained increase in NE-induced superoxide production was primarily mediated via α-adrenergic receptors, preferentially α2-receptors. The NADPH oxidase blocker diphenylene iodonium and protein kinase C inhibitor staurosporine significantly attenuated NE-induced increases in superoxide production. Importantly, NE treatment increased CD14⁺ monocyte-endothelial cell adhesion. These findings indicate for the first time that NE increases superoxide production in freshly isolated primary human PBMCs via NADPH oxidase through α-adrenergic receptors; an effect facilitating monocyte adhesion to the endothelium.