Antioxidant enzyme activity was measured in salt tolerant callus tissue derived from the cultivar Coker 312 over an 8 hour period following treatment with either 250 mM NaCl, 2 µM staurosporine, 2µ M K252a, 5 µM TMB-8, 100 nM A23187, 250 mM NaCl + 2µ M staurosporine, 250 mM NaCl + 2µ M K252a, 250 mM NaCl + 5 µM TMB-8, and 250 mM NaCl + 5 µM TMB-8 + 100 nM A23187. Staurosporine and K252a have been reported to inhibit plant protein kinase activity. TMB-8 is a putative calcium channel blocker, and A23187 is a calcium ionophore. NaCl induced an up-regulation of catalase, peroxidase, ascorbate peroxidase, and glutathione reductase activities within 1 hour after treatment. The K252a and TMB-8 treatments completely inhibited the NaCl stress-induced increases in the activities of all four enzymes. At the concentration used in this experiment, staurosporine did not completely inhibit the NaCl-induced increases in antioxidant activity; however, it either reduced the responses of ascorbate peroxidase, glutathione reductase, and peroxidase or delayed the response of catalase. Treatment with A23187 increased antioxidant activity in the NaCl-tolerant controls, and treatment with TMB-8 + A23187 increased antioxidant activity in both the controls and the NaCl-stressed callus. These data support the hypothesis that protein kinases are involved in the signal transduction pathway associated with the NaCl-induced upregulation of antioxidant activity and that these protein kinases may be regulated by calcium.