||In plants, excess reactive oxygen species are toxic molecules induced under environmental stresses, including pathogen invasions and abiotic stresses. Many anti-oxidant defense systems have been reported to require NADPH as an important reducing energy equivalent. However, the sources of NADPH and the molecular mechanisms of maintaining cytoplasmic redox balance are unclear. Here, we report the biological function of a putative cytoplasmic NADH kinase (NADK3) in several abiotic stress responses in Arabidopsis. We found that cytoplasmic NADPH is provided mostly by the product of the NADK3 gene in Arabidopsis. Expression of he NADK3 gene is responsive to abscisic acid (ABA) and abiotic stress conditions, including methyl violgen (MV), high salinity and osmotic shock. An NADK3 null mutant showed hypersensitivity to oxidative stress in both seed germination and seedling growth. Seed germination of the mutant plants also showed increased sensitivity to ABA, salt and mannitol. Furthermore, stress-related target genes were identified as upregulated in the mutant by mannitol and MV. Our study indicates that this cytoplasmic NADH kinase, a key source of the cellular reductant NADPH, is required for various abiotic stress responses.