Aminotriazole(AT)-induced changes in growth, hydrogen peroxide content and activities of H2O2-scavenging antioxidant enzymes were investigated in the growing leaves ofArabidopsis plants (Arabidopsis thaliana cv Columbia). Catalase activity of rosette leaves was reduced by 65% with an application of 0.1 mM AT (a herbicide known as a catalase inhibitor), whereas the leaf growth and H2O2 content were almost unaffected. However, an approximate 1.6 to 2-fold increase in cytosolic ascorbate peroxidase (APX) activity concomitant with a substantial activation of glutathione reductase (GR) (approx. 22% increase) was observed during leaf growth in the presence of 0.1 mM AT. The activity of cytosolic APX in leaves was also increased by 1.8-fold with an application of exogenous 2 mM paraquat (an inducer of H2O2 production in plant cells) in the absence of AT. These results collectively suggest that (a) cytosolic APX and GR operate to activate an ascorbate-glutathione cycle for the removal of H2O2 under severe catalase deactivation, and (b) the expression of APX seems to be regulated by a change of the endogenous H2O2 level in leaf cells.