Callus tissue from a control and an NaCl-tolerant cell line was transferred to culture tubes containing media amended with 0 NaCl (NaCl-sensitive control) or 150 mM NaCl (NaCl-tolerant control) and media amended with 250 mM NaCl, 0.1 mg/L ABA, 0.01% H₂O₂, or 2.5 mM xanthine and 0.05 units of xanthine oxidase. The callus tissue was harvested at 30 min, 1 hr, 2 hr, 4 hr, and 8 hr intervals and analyzed for antioxidant enzyme activity. The NaCl-acclimated callus (NaCl-tolerant cell line) initiated an upregulation of antioxidant activity much earlier than the control callus. While NaCl, ABA, H₂O₂, and O₂^-. all initiated upregulation of superoxide dismutase (SOD), ascorbate peroxidase (AP), glutathione reductase (GR), and peroxidase in the NaCl-tolerant cell line, the increases resulting from ABA and O₂^-. either closely matched or preceded the increases observed with the NaCl treatment. Increase in enzyme activity resulting from H₂O₂ treatment were generally much later than those observed with the other treatments. These data suggest a possible mechanism whereby in NaCl stress, the excess Na⁺ and/or Cl^- results in electron leakage and production of O₂ which serves as the signal molecule for the induction of stress metabolites including ABA. The elevated ABA levels may then result in the enhanced induction of antioxidant enzyme encoding genes.