With the aim to assess the effect of possible O2*- generation during an oxidative burst on O3 reduction in mesophyll cell walls due to the reaction O2*- + O3 --> O3*- + O2 and subsequent formation of *OH, 03 flow through this sequence was compared with O2*- flow through the competitive sequences where H2O2 is formed. The two-electron reduction of O3 via the direct reaction with ascorbate was also considered. The calculations were exemplified in an experiment where Phaseolus vulgaris L. leaves were exposed to 530 nl O3 l(-1) in air for 3.5 h. During the exposure, H2O2 was assumed to be generated at peak rates observed in pathogen-elicited cell suspensions. O3 reduction through reaction with O2*- was 25-44% of O3 detoxified in the direct reaction with ascorbate. More than 99% of O2*- was reduced to H2O2 via spontaneous disproportionation and reduction with ascorbate, the disproportionation prevailing at pH 5 and reduction at the expense of ascorbate at pH 7. H2O2 was estimated to be channelled mostly to the peroxidase-catalysed scavenging reaction. Calculated steady state H2O2 concentrations were 40-80 microM. It is concluded that generation of H2O2 at the postulated rate was too high and that the generation of O2*- during an oxidative burst is ineffective in reducing O3 through the network of reactive oxygen species. Superoxide dismutase induction in the cell wall under O3 is discussed.