Vascular endothelial cell activation by cytokines and other pro-inflammatory mediators is and initial event in atherosclerosis and in other vascular diseases. Simvastatin, a HMG-CoA reductase inhibitor, suppressed both tumor necrosis factor $(TNF)-{alpha}$- and angiotensin (Ang) Il-induced monocyte adhesion to endothelial cells (an initial step in vascular inflammation) and reactive oxygen species (ROS) production. Diphenyleneiodonium and apocynin, both NADPH oxidase inhibitors, also suppressed TNF-${alpha}$-induced ROS and monocyte-endothelial cell adhesion, demonstrating that $TNF-{alpha}$-induced monocyte adhesion is mediated through ROS produced by NADPH oxidase activation. Furthermore, exogenously applied mevalonate or geranylgeranylpyrophosphate in combination with simvastatin completely prevented the inhibitory effects of simvastatin on ROS generation and monocyte-endothelial cell adhesion by $TNF-{alpha}$ and Ang Il. These results suggest that monocyte adhesion to endothelial cells induced by $TNF-{alpha}$ or Ang Il is mediated via the geranylgeranyl isoprenoid-dependent generation of ROS, and that this is inhibited by simvastatin.