This study investigated mechanisms underlying native low-density lipoprotein (LDL)-stimulated proliferation of human vascular smooth muscle cells (VSMC). Experiments were performed to determine whether native LDL affects reactive oxygen species (ROS) formation and activity of extracellular signal-regulated kinase 1/2 (ERK1/2), and whether redox-sensitive pathways contribute to LDL-induced cell proliferation. Native LDL (100 μg/mL, 24 hours) increased cell proliferation (to 303 to 388% of control, P <0.0001) as determined by [methyl- ³ H] thymidine incorporation. This effect was completely blocked either by the antioxidants N-acetylcysteine, Tiron, or nordihydroguaiaretic acid; the flavin-inhibitor diphenylene iodonium; or superoxide dismutase (all P <0.0001), and partly blocked by ERK-inhibitor PD98059 or meclofenamate ( P <0.01). Exposure of VSMC to native LDL for 20 minutes stimulated ROS formation, measured by dichlorodihydrofluorescein oxidation, and increased ERK1/2 activity by 3.1-fold ( P <0.001). The latter effect was sensitive to MEK1/2 inhibitor PD98059 and Tiron ( P <0.001), and in part to N-acetylcysteine or diphenylene iodonium ( P <0.05). These results demonstrate that native LDL induces acute formation of ROS and subsequent activation of redox-sensitive ERK 1/2 mitogen-activated protein kinases, pathways that appear to be important for mitogenic signaling of native LDL in human vascular smooth muscle cells.