Human umbilical vein endothelial cells (HUVECs) have played a major role as a model system for the study of the regulation of endothelial cell function and the role of the endothelium in the response of the blood vessel wall to stretch, shear forces, and the development of atherosclerotic plaques and angiogenesis. Here, we use HUVECs and human microvascular endothelial cells to study the role of the HMG-CoA reductase inhibitor, simvastatin, and the small GTP-binding protein Rho in the regulation of angiogenesis. Simvastatin inhibited angiogenesis in response to FGF-2 in the corneal pocket assay of the mouse and in vascular endothelial growth factor (VEGF)-stimulated angiogenesis in the chick chorioallontoic membrane. Furthermore, simvastatin inhibited VEGF-stimulated tube formation by human dermal microvascular endothelial cells and the formation of honeycomb-like structures by HUVECs. The effect was dose-dependent and was not secondary to apoptosis. Geranylgeranyl-pyrophosphate (GGPP), a product of the cholesterol metabolic pathway that serves as a substrate for the posttranslational lipidation of RhoA, was required for membrane localization, but not farnesylpyrophosphate (FPP), the substrate for the lipidation of Ras. Furthermore, GGTI, a specific inhibitor of GGPP, mimicked the effect of simvastatin of tube formation and the formation of honeycombs whereas FTI, a specific inhibitor of the farnesylation of Ras, had no effect. Adenoviral expression of a DN-RhoA mutant mimicked the effect of simvastatin on tube formation and the formation of honeycombs, whereas a dominant activating mutant of RhoA reversed the effect of simvastatin on tube formation. Finally, simvastatin interfered with the membrane localization of RhoA with a dose-dependence similar to that for the inhibition of tube formation. Simvastatin also inhibited the VEGFstimulated phosphorylation of the VEGF receptor KDR, and the tyrosine kinase FAK, which plays a role in cell migration. These data demonstrate that simvastatin interfered with angiogenesis via the inhibition of RhoA. Data supporting a role for angiogenesis in the development and growth of atherosclerotic plaques suggest that this antiangiogenic effect of Statins might prevent the progression of atherosclerosis via the inhibition of plaque angiogenesis.