The oxidative cleavage of cis-epoxycarotenoids catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED) is considered to be the ratelimitingstep in abscisic acid (ABA) biosynthesis. Here we demonstrate that the expression of AhNCED1 gene in peanut plants is significantlyup-regulated by dehydration and high salinity. The AhNCED1 transcript and endogenous ABA both accumulate predominantlyin leaves and stems of peanut in response to dehydration. The considerable up-regulation of AhNCED1 expression by exogenous applicationof ABA suggests a positive feedback control of ABA biosynthesis in peanut. NAA at high concentration increases ABA biosynthesisin peanut plants through up-regulation of the AhNCED1 gene expression. The constitutive expression of the AhNCED1 gene inwild-type Arabidopsis results in an increase of ABA accumulation in transgenic plants in response to drought stress. Ectopic expression ofAhNCED1 gene in 129B08/nced3 mutant Arabidopsis (with impaired AtNCED3 gene involved in ABA biosynthesis under water stress)driven by the AtNCED3 promoter restores its ability to accumulate ABA during drought stress, and reverts its hypersensitivity to nonionicosmotic stress and soil drought. These results indicate that the expression of AhNCED1 gene plays an important role in theregulation of ABA level during water stress, and that water-stress tolerance of Arabidopsis plants can be improved by ectopic expressionof the AhNCED1 gene causing accumulation of endogenous ABA.