In plants, environmental adversity often leads to the formation of highly reactive oxygen radicals. Since resistance to such conditions may be correlated with the activity of enzymes involved in oxygen detoxification, we have generated transgenic tobacco plants which express elevated levels of manganese superoxide dismutase (MnSOD) within their chloroplasts or mitochondria. Leaf discs of these plants have been analyzed in conditions in which oxidative stress was generated preferentially within one or the other organelle. It was found that high level overproduction of MnSOD in the corresponding subcellular location could significantly reduce the amount of cellular damage which would normally occur. In contrast, small increases in MnSOD activity were deleterious under some conditions. A generally applicable model correlating the consequences of SOD with the magnitude of its expression is presented.