Oxidative stress may be involved in the development of vascular complications associated with diabetes; however, the molecular mechanism responsible for increased production of free radicals in diabetes remains uncertain. Therefore, we examined whether acute hyperinsulinemia increases the production of free radicals and whether this condition affects proliferative extracellular signal-regulated kinase (ERK-1 and -2) signaling in human fibroblasts in vitro. Insulin treatment significantly increased intracellular superoxide anion (O(2)(-)) production, an effect completely abolished by Tiron, a cell-permeable superoxide dismutase (SOD) mimetic and by polyethylene glycol (PEG)-SOD, but not by PEG catalase. Furthermore, insulin-induced O(2)(-) production was attenuated by the NAD(P)H inhibitor apocynin, but not by rotenone or oxypurinol. Inhibition of the phosphatidylinositol 3'-kinase (PI 3'-kinase) pathway with LY294002 blocked insulin-stimulated O(2)(-) production, suggesting a direct involvement of PI 3'-kinase in the activation of NAD(P)H oxidase. The insulin-induced free radical production led to membranous translocation of p47phox and markedly enhanced ERK-1 and -2 activation in human fibroblasts. In conclusion, these findings provided direct evidence that elevated insulin levels generate O(2)(-) by an NAD(P)H-dependent mechanism that involves the activation of PI 3'-kinase and stimulates ERK-1- and ERK-2-dependent pathways. This effect of insulin may contribute to the pathogenesis and progression of cardiovascular disease in the insulin resistance syndrome.