The abnormal proliferation and migration of vascular smooth muscle cell (VSMC), which is triggered by various external stimuli, contributes importantly to the pathogenesis of atherosclerosis and restenosis. Recent studies indicate that the endoplasmic reticulum (ER) stress is intensively involved in the pathophysiological changes of VSMCs by various stimuli. However, the direct effects of ER stress on VSMC proliferation and migration remain unknown. In this study, we found that pretreatment with tunicamycin (Tm), an ER stress inducer, significantly inhibited platelet-derived growth factor (PDGF)-BB-induced VSMC proliferation and migration in a dose-dependent manner without causing significant apoptosis. Tm stimulated the expression of the antioxidant gene heme oxygenase-1 (HO-1) both at the transcriptional and translational levels, while reducing phosphorylation and activation of mitogen-activated protein (MAP) kinases. The negative regulative effects of Tm were associated with a decrease in cyclins and cyclin-dependent kinases (CDKs) activation. More importantly, HO-1 siRNA partially abolished the beneficial effects of Tm on VSMCs. These results indicate that Tm-induced ER stress provides protection against the abnormal VSMC activation by PDGF-BB, which may be mediated by the induction of HO-1 and blockade of cell cycle reentry. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc.