Ethylenediurea (EDU), N- [2-(2-oxo-1-imidazolidinyl) ethyl]-N -phenylurea is known to prevent ozone (O 3 ) damage to leaf tissues. However, the mechanisms of protection are unclear. We tested the hypothesis that EDU protects against O 3 damage by scavenging hydroxyl free radicals (OH). An in vitro study involving the use of high-performance liquid chromatography equipped with an electrochemical detector (HPLC-EC) showed that EDU does not serve as an antioxidant to remove OH free radicals. Effects of O 3 and EDU (soil drench) on leaf antioxidant scavenger systems (AOSS) were also studied. The first fully expanded trifoliate leaves of O 3 -sensitive snap bean (Phaseolus vulgaris cv. Bush Blue Lake 290) was examined. Measurements were made before and after a single O 3 exposure (0.30 μl l - 1 O 3 for 3 h). Pretreatment with EDU 48 h before exposure protected against O 3 -induced necrosis and chlorosis. EDU pretreatments did not alter superoxide dismutase (SOD), guaiacol-peroxidase (GPX), ascorbate peroxidase (APX) and glutathione reductase (GR) activities. However, O 3 -fumigated plants (no EDU) showed elevated SOD activity with decreased GR activity. EDU-treated plants exposed to O 3 stress showed no measurable loss of GR activity. These tissues maintained high levels of total glutathione [i.e. reduced glutathione (GSH) + oxidized glutathione (GSSG)] contents, and had higher GSH/GSSG ratios than the controls at the end of 3 h exposure to O 3 . These data suggest that EDU protection against O 3 damage in plants do not necessarily involve the direct stimulation or induction of antioxidative enzyme defense mechanisms. Instead, protection may result from a more general retention of chlorophyll and maintenance of GR and GSH levels during O 3 exposure.