Effects of environmental stress caused by elevated levels of carbon dioxide on the level of glutathione S-transferase (GST) activity were investigated in leaf tissues of various plant species growing in the proximity of two natural CO2 springs in central Italy. Leaf tissues of Phragmites australis grown in elevated CO2 concentrations at the CO2 spring of Bossoleto contained significantly reduced GST levels as compared to control plants. Similar, but statistically not significant changes in GST levels of Chenopodium album, Quercus ilex, Quercus pubescens and Quercus robur were detected.