AbstractTumour angiogenesis is mediated by increased levels of vascular endothelial growth factor (VEGF). We have studied the mechanism by which endogenous activation of Rho oncoproteins regulates VEGF expression in COS‐7 and NIH3T3 cells. We carried out transient and stable transfection with constitutively activated rhoA, rac1, and cdc42 mutants in COS‐7 and NIH3T3 cells, respectively in the absence of external stimuli. Western blot and inmunohistochemistry assays of those cells revealed increased VEGF protein expression. Cotransfection with constitutively activated rhoA, rac1, and cdc42 mutants and a VEGF promoter‐reporter construct showed an increase in VEGF promoter transcriptional activity induced by Rho oncoproteins in COS‐7 and NIH3T3. c‐Jun kinase had been described as a MAPK involved in Rho oncoproteins pathways. Interestingly, we found that c‐Jun kinase chemical inhibition as well as transient transactivation assays using dominant negative c‐Jun kinase mutant abolished the VEGF promoter transcriptional induction by Rac1 and Cdc42 but not by RhoA. These findings indicate that Rho oncoprotein endogenously activated regulates VEGF expression through a transcriptional mechanism, and that the c‐Jun kinase activity is a mediator in the expression of VEGF induced by Rac1 and Cdc42 oncoproteins, but not of that induced by RhoA. J. Cell. Biochem. 98: 650–660, 2006. © 2006 Wiley‐Liss, Inc.