Ozone-tolerant Bel B and ozone-sensitive Bel W3 tobacco cultivars were subjected to acute ozone fumigation (200 p.p.b. for 3 h) and the subcellular localization of H2O2 was then studied. H2O2 accumulated on the cell walls and plasma membrane of both cultivars but the accumulation pattern differed greatly. H2O2 production was high in both cultivars immediately after fumigation, but, in the tolerant Bel B cultivar, after 7 h was only detected in some spongy cells adjacent to epidermal cells. Instead, in the sensitive Bel W3 cultivar, accumulation was still abundant in the cell walls of palisade, spongy and epidermal cells at this time. Significant changes in apoplastic ascorbate pool were noted in both cultivars in the first hours after fumigation. As the reduced ascorbate content remained unchanged, the marked increase in total ascorbate must have originated from the striking increase in dehydroascorbate, particularly in the ozone-sensitive Bel W3. Exposure of plants to ozone resulted in a marked transient increase in both free and conjugated salicylic acid (SA) as well as an increase in the activity of benzoic acid 2-hydroxylase which catalyses SA biosynthesis. SA induction differed greatly in the two cultivars, in that: (1) SA accumulation was far greater in the ozone-sensitive Bel W3 cv. and (2) the maximum SA peak was delayed in Bel W3 and observed only 7 h after fumigation ended. These results suggest that a high SA content, as documented in the ozone-sensitive Bel W3 cultivar, could trigger the production of ROS with subsequent SA-mediated cell-death.