In sunflower plants exposed to 150 nl L-1 of O3 (4 h day-1 for 4 days) an enhancement in the activity of APx was found at both the apoplastic and symplastic levels, while stromal and thylakoid-bound chloroplastic APx activity did not change. PAGE analysis confirmed the presence of APx activity in the apoplast and revealed different APx isoenzyme patterns between control and O3-treated plants in either apoplastic and cytoplasmic samples. Immunoreaction with a cytosolic APx antibody revealed, at both the apoplastic and symplastic levels, only one band, more evident in the O3-treated than in the control plants. These results support the hypothesis of a cytoplasmic origin for the apoplastic isoform. Thus, sunflower plants seem to adapt to O3-induced oxidative stress through the induction of genes coding for APx proteins whose enzymatic activity enhancement at the apoplastic and cytosolic levels may act as a biochemical defence line against ozone pollution damage.