Superoxide detection in time course studies showed increases in superoxide radicals in response to exposure to NaCl and paraquat. Both control callus and 150 mM NaCl acclimated callus were treated with paraquat and NaCl. Both the paraquat and NaCl treatments resulted in significant increases in superoxide levels, but the paraquat induced increases were significantly higher than the NaCl-induced increases. In all cases the NaCl acclimated callus tissue exhibited significantly lower levels of superoxide production than the control callus. Increases in superoxide levels directly correspond with earlier observations of increases in antioxidant enzyme levels in response to NaCl and paraquat stress.