An oxidative chain reaction of sulfite initiated by the superoxide ion produced in the Mehler reaction has been implicated in the damage of plants exposed to sulfur dioxide. The toxicity of SO(2) may be alleviated by free radical scavenging systems acting to terminate this chain reaction. Hence, the relative sensitivity of plants to SO(2) toxicity could depend on differences in the responses of the levels of antioxidant metabolites and enzymes. The effect of SO(2) exposure on glutathione and ascorbic acid contents, glutathione reductase, and superoxide dismutase activities was assayed in two cultivars (Progress, Nugget) of pea (Pisum sativum L.) in which apparent photosynthesis showed a differential sensitivity to 0.8 microliter per liter SO(2) (R. Alscher, J. L. Bower, W. Zipfel [1987] J Exp Bot 38:99-108). Total and reduced glutathione increased more rapidly and to a greater extent in the insensitive Progress than in the sensitive Nugget, as did glutathione reductase activities. Superoxide dismutase activities increased significantly in Progress, whereas no such change was observed in Nugget as a result of SO(2) exposure. This increase in superoxide dismutase activity was observed at 210 minutes after 0.8 microliter per liter SO(2) concentration had been reached, in marked contrast to the increases in reduced glutathione content and glutathione reductase activity, which were apparent at the 90 minute time point. These data suggest that one basis for the relative insensitivity of the apparent photosynthesis of the pea cultivar Progress to SO(2) is the enhanced response of glutathione reductase, superoxide dismutase activities, and glutathione content.