Two varieties of tobacco (Nicotiana tabacum var PBD6 and var SR1) were used to generate transgenic lines overexpressing Mn-superoxide dismutase (MnSOD) in the chloroplasts. The overexpressed MnSOD suppresses the activity of those SODs (endogenous MnSOD and chloroplastic and cytosolic Cu/ZnSOD) that are prominent in young leaves but disappear largely or completely during aging of the leaves. The transgenic and control plants were grown at different light intensities and were then assayed for oxygen radical stress tolerance in leaf disc assays and for abundance of antioxidant enzymes and substrates in leaves. Transgenic plants had an enhanced resistance to methylviologen (MV), compared with control plants, only after growth at high light intensities. In both varieties the activities of FeSOD, ascorbate peroxidase, dehydroascorbate reductase, and monodehydroascorbate reductase and the concentrations of glutathione and ascorbate (all expressed on a chlorophyll basis) increased with increasing light intensity during growth. Most of these components were correlated with MV tolerance. It is argued that SOD overexpression leads to enhancement of the tolerance to MV-dependent oxidative stress only if one or more of these components is also present at high levels. Furthermore, the results suggest that in var SR1 the overexpressed MnSOD enhances primarily the stromal antioxidant system.