Freshly isolated protoplasts from pea leaves were used to investigate the responses of antioxidant enzymes to oxidative stress. Two cultivars, Progress (tolerant) and Nugget (sensitive), that have differing resistance with respect to oxidative stress at the whole plant level were used. Sulfite and the superoxide generating herbicide, paraquat, were used as the oxidants. Final sulfite concentrations during photosynthetic incubations ranged from 1.5 mM to 30.0 mM. During the polarographic estimation of photosynthesis, CO₂-dependent O₂ evolution did not decrease. At sulfite concentrations of 3.0 mM or less, light-dependent O₂ evolution increased and was probably due to a concomitant SO₂-dependent O₂ evolution. Photosynthesis determined as ¹⁴CO₂ fixation was not increased at these low concentrations of sulfite. Concentrations greater than 7 mM = sulfite inhibited photosynthetic ¹⁴CO₂ fixation. No difference in these responses was found between the two cultivars. At 0.1 µM paraquat, the relative resistance to oxidative stress was reversed compared to previous studies at the whole plant level. With the tolerant cultivar, activity of the plastid antioxidant enzyme, glutathione reductase, increased after a three-hour exposure. Changes in the steady state level of glutathione reductase protein, as judged by immunoblots, did not correlate with the observed changes in enzyme activity. No change in the de novo synthesis of glutathione reductase occurred over the same period as a consequence of paraquat application. A mechanism, unrelated to oxygen free radical scavenging, may contribute to the relative tolerance to low concentrations of paraquat. On the other hand, after an eight-hour exposure to 0.1 mM PQ in the presence of Gamborg’s basal salts, superoxide dismutase activity of Progress protoplasts was enhanced 288% above the preexposure levels while glutathione reductase activity decreased 70% and ascorbate peroxidase activity decreased 90%. The relationship of these changes to oxidative damage to the photosynthetic machinery remains to be assessed. / Ph. D.