The mechanisms by which the maize antioxidant Cat1 gene responds to abscisic acid (ABA) and osmotic stress have been investigated. Results show that during late embryogenesis, Cat1 expression in vivo is independent of endogenous ABA levels. However, exogenously applied ABA significantly enhances Cat1 expression. Transient assays using particle bombardment show that the proximal ABRE2 element on the Cat1 promoter is responsible for the induction of Cat1 expression by ABA. We further show that ABA induces the expression of Cat1 via the interaction between ABRE2 and one of its binding proteins, CBF1 (Cat1 binding factor 1). Using ABA-deficient mutant embryos, we show that osmotic stress induces Cat1 expression through two alternate signal transduction pathways: an ABA signaling pathway leading to the interaction between the ABRE2 motif and CBF1, and a pathway via the interaction of ABRE2 and CBF2 (Cat1 binding factor 2) that is independent of ABA. The data presented clearly suggest that hydrogen peroxide (H2O2) plays an important intermediary role in the ABA signal transduction pathway leading to the induction of the Cat1 gene.