Previously, we demonstrated that the level of intracellular O-2(-) is increased by low-density lipoprotein (LDL) in human aortic smooth muscle cells (HASMCs). The exact role of O-2(-) in the LDL-induced proliferation of HASMCs, however, has not been determined. In this study, we found that the increase in the concentrations of intracellular O-2(-) induced by native and oxidized LDL increased SMC-nitric oxide (NO) uptake rate. Moreover, the treatment of HASMCs with diethyldithiocarbamate (DETC), a superoxide dismutase inhibitor, significantly increased NO uptake rate owing to the increase in intracellular O-2(-) concentrations. Although native and oxidized LDL decreased soluble guanylyl cyclase (sGQ protein content, they still caused a net increase in cyclic GMP production in HASMCs. In addition, when cyclic GMP production was normalized by sGC protein content and NO uptake rate, it was found to be positively dependent on the level of intracellular H2O2. Finally, we simulated cell proliferation stimulated by native and oxidized LDL as a linear function of intracellular O-2(-) and H2O2 concentrations, demonstrating that O-2(-) negatively modulated the native and oxidized LDL-stimulated HASMC proliferation through the increase in NO uptake rate.