Spinach (Spinacia oleracea L.), broad bean (Viciafaba L.) and beech (Fagus sylvatica L.) plants were exposed to ozone at concentrations often measured in air during the summer months (120-300 pg. m-3) and antioxidants were determined in the leaf tissue and in the aqueous phase of the cell wall, the apoplasm. Concentrations of both reduced ascorbate (AA) and its oxidized form, dehydroascorbate (DHA), showed the tendency to increase transiently in the apoplasm of spinach leaves 6-24 h after starting fumigation with ozone. In beech leaves, apoplasmic AA and DHA increased 3-7 d after beginning of treatment. At the very high concentration of 1600 lagO3.m -3, an increase of apoplasmic AA was already measured after 1 d in beech leaves. Apparently, spinach and beech leaves respond to oxidative stress by increasing AA transport into the apoplasm and by accelerating DHA export. In contrast to these observations, DHA accumulated during 3 d of fumigation with only 120 lag O3"m -3 in the apoplasm of broad bean leaves, while AA contents did not increase. After termination of fumigation, the extracellular redox state of ascorbate normalized within 1 d. Glutathione could not be detected in the apoplasm of any of the three leaf species, lntracellular AA changed its redox state in response to exposure to elevated concentrations of ozone. After 4-6 weeks of fumigation with 200-300 lag 03" m-3 an increase of intracellular DHA was measured in beech leaves. At the same time, chlorophyll contents decreased and characteristic symptoms of ozone damage could be observed. However, no significant change in the redox state of apoplasmic ascorbate could be detected in beech leaves. Evidently, detoxification of ozone by apoplasmic AA was insufficient to protect the leaf tissue. Fumigation with a high ozone