Small GTPase Rho and Rho-kinase, the target protein of Rho, play an important role in atherosclerosis. In diabetic macroangiopathy, one of the major pathogenic changes is the migration of vascular smooth muscle cells (SMCs). Platelet-derived growth factor (PDGF) is known to stimulate the migration of SMCs. In the current study, we have investigated the involvement of the Rho/Rho-kinase pathway in the increased migration of cultured human aortic SMCs under a high glucose condition. PDGF stimulated the activation and the protein level of Rho. The protein level of PDGF receptor-beta (PDGFR-beta) was increased under the high glucose condition concomitant with the increased protein level and activation of Rho. The increased protein level and activity of Rho were suppressed by an anti-PDGF neutralizing antibody or a PDGFR-beta inhibitor, AG1433, under the high glucose condition. Furthermore, high glucose significantly increased the migration of SMCs. A specific inhibitor of Rho-kinase, Y-27632, or anti-PDGF neutralizing antibody inhibited increased migration of SMCs under the high glucose condition. The protein levels of Rho were increased in aortae of diabetic rats, which were abolished by the treatment of Imatinib, the inhibitor of PDGFR. These observations indicate that the upregulation of the PDGFR-beta / Rho / Rho-kinase pathway increases the migration of SMCs under the high glucose condition. The inhibition of Rho/Rho-kinase may be a new target for the treatment of diabetic macroangiopathy.