The aim of this work was to examine the correspondence between apoplastic\symplastic antioxidant status and previously reported plant age-related shifts in the ozone (O $ ) resistance of Plantago major L. Seed-grown plants were fumigated in duplicate controlled environment chambers with charcoal\Purafil2-filtered air (CFA) or CFA plus 70 nmol mol −" O $ for 7 h d −" over a 42 d period. Measurements of stomatal conductance and antioxidants were made after 14, 28 and 42 d fumigation, on leaves at an equivalent stage of development (youngest fully expanded leaf, measured c. 9 d after emergence). Ozone exposure resulted in a similar decline in stomatal conductance across plant ages, indicating that increases in O $ resistance with plant age were mediated through changes in the tolerance of leaf tissue rather than enhanced pollutant exclusion. Leaf apoplastic washing fluid was found to contain ' unspecific ' peroxidase, ascorbate peroxidase, superoxide dismutase and ascorbate, but not glutathione and the enzymes required to facilitate the regeneration of ascorbate from its oxidized forms. A weak induction in the activity of certain symplastic antioxidants was found after 14 d O $ fumigation, despite a lack of visible symptoms of injury, but shifts in symplastic antioxidant enzyme activity were not consistent with previously observed increases in resistance to O $ with plant age. By contrast, changes in ' unspecific ' peroxidase activity and in the small pool of ascorbate in the leaf apoplast were found to accompany age-related shifts in O $ resistance. It is concluded that constituents of the leaf apoplast may constitute a potentially important front line defence against O $ .