Callus tissue from a NaCl-tolerant cell line was transferred to culture tubes containing 150 mM NaCl (NaCl-tolerant control) and media containing 150 mM NaCl + 0.2 mM fluridone. Following a 2-hour preincubation, the tubes containing 150 mM NaCl and the tubes containing 150 mM NaCl + 0.2 mM fluridone were amended with NaCl to a final concentration of 250 mM NaCl, 0.1 mg/L ABA, 0.1 mM paraquat, or 0.01% H2O2. The callus tissue was harvested at 30 min, 1 hr, 2 hr, 4 hr, and 8 hr intervals and analyzed for ascorbate reductase (AP) and glutathione reductase (GR) activity. All treatments except hydrogen peroxide and hydrogen peroxide plus fluridone induced significant increases in AP activity above the control activity levels within 1 hour after treatment. Pretreatment with fluridone failed to suppress the increase in AP activity in all treatments except when used with hydrogen peroxide which did not induce an increase AP activity when used alone. Except for fluridone and hydrogen peroxide plus fluridone, all treatments resulted in significant increases in GR activity above the control levels within one hour after treatment. In contrast to AP, pretreatment with fluridone suppressed or significantly delayed the increase in GR activity in all but the paraquat treated tissues. These data suggests that the stress-induced increases in the activities of GR and AP result from different signaling mechanisms. AP activity appears to be rapidly induced by almost any stress metabolite completely independent of ABA, while stress-induced increases in GR activity are generally slower and partially, at least in the case of NaCl stress, associated with ABA concentrations.