Aminotriazole(AT)-induced changes in growth, hydrogen peroxide content and activities of H2O2-scavenging antioxidant enzymes were investigated in the growing leaves of Arabidopsis plants (Arabidopsis thaliana cv Columbia). Catalase activity of rosette leaves was reduced by 65% with an application of 0.1mM 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 0.1mM AT. The activity of cytosolic APX in leaves was also increased by 1.8-fold with an application of exogenous 2mM 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.