The present study investigated the role of reactive oxygen species (ROS) in activation of nuclear factor of activated T cells (NFAT), a pivotal transcription factor responsible for regulation of cytokines, by vanadium in mouse embryo fibroblast PW cells or mouse epidermal Cl 41 cells. Exposure of cells to vanadium led to the transactivation of NFAT in a time- and dose-dependent manner. Scavenging of vanadium-induced H(2)O(2) with N-acety-L-cyteine (a general antioxidant) or catalase (a specific H(2)O(2) inhibitor) or the chelation of vanadate with deferoxamine, resulted in inhibition of NFAT activation. In contrast, an increase in H(2)O(2) generation by the addition of superoxide dismutase or NADPH enhanced vanadium-induced NFAT activation. This vanadate-mediated H(2)O(2) generation was verified by both electron spin resonance and fluorescence staining assay. These results demonstrate that H(2)O(2) plays an important role in vanadium-induced NFAT transactivation in two different cell types. Furthermore, pretreatment of cells with nifedipine, a calcium channel blocker, inhibited vanadium-induced NFAT activation, whereas and ionomycin, two calcium ionophores, had synergistic effects with vanadium for NFAT induction. Incubation of cells with cyclosporin A (CsA), a pharmacological inhibitor of the phosphatase calcineurin, blocked vanadium-induced NFAT activation. All data show that vanadium induces NFAT activation not only through a calcium-dependent and CsA-sensitive pathway but also involved H(2)O(2) generation, suggesting that H(2)O(2) may be involved in activation of calcium-calcineurin pathways for NFAT activation caused by vanadium exposure.