NADPH oxidases are important sources of vascular superoxide, which has been linked to the pathogenesis of atherosclerosis. Previously we demonstrated that the Nox4 subunit of NADPH oxidase is a critical catalytic component for superoxide production in quiescent vascular smooth muscle cells. In this study we sought to determine the role of Nox4 in superoxide production in human aortic smooth muscle cells (AoSMC) and embryonic kidney (HEK293) cells under proinflammatory conditions. Incubation with tumor necrosis factor-alpha (TNF-alpha, 10 ng/ml) for 12 h increased superoxide production in both cell types, whereas angiotensin II, platelet-derived growth factor or interleukin-1beta had little effects. Superoxide production was completely abolished by the NADPH oxidase inhibitors diphenyline iodonium and apocynin, but not by inhibitors of xanthine oxidase, nitric oxide synthase or mitochondrial electron transport. TNF-alpha upregulated the expression of Nox4 in AoSMC at both message and protein levels, while Nox1 and Nox2 were unchanged. In contrast, upregulation of Nox2 appeared to mediate the enhanced superoxide production by TNF-alpha in HEK293 cells. We suggest that Nox4 may be involved in increased superoxide generation in vascular smooth muscle cells under proinflammatory conditions.