The present study was performed to see the physiological role of cytosolic ascorbate peroxidase (APX) and its relationship to other enzymes involved in the H2O2 scavenging metabolism, and also to elucidate the regulation of APX expression in dark-grown radish (Raphanus sativus L. cv Taiwang) cotyledons. To do so, 3-amino-1, 2, 4-triazole (aminotriazole), a known specific inhibitor of catalase, was used to simulate a catalase-deficient phenomenon in cotyledons. aminotriazole, in very low concetration (10-4 M), inhibited remarkably the development of catalase activity in cotyledons during dark germination. This inhibition of catalase by aminotriazole, however, did not result in any significant changes in the growth response and the H2O2 level of developing cotyledons. In addition, the development of guaiacol peroxidase (GPX) activity was also not significantly affected. Unlike GPX, cytosolic APX activity was induced radpily and reached a 1.7-fold increase in aminotriazole treated cotyledons at day 7 after germination. However, in vitro incubation of cytosolic APX preparation from cotyledons with aminotriazole did not result in any significant change in activity. One cytosolic APX isozyme (APXa) band involved in this APX activation was predominantly intensified in a native polyacrylamide gel by activity staining assay. This means that this APXa isozymes seems to play a key role in the expression of cytosolic APX activity. On the other hand, 2-day-old control seedlings treated with exogenous 1mM H2O2 for 1 h showed a significant increase of cytosolic APX activity even in the absence of aminotriazole. Also, 2 $mu$M cycloheximide treatment substantiallyinhibited the increase of APX activity due to aminotriazole. Based on these results, we suggest that a radish cytosolic APX could probably be substituted for catalase in H2O2 removal and that the expression of APX seems to be regulated by a change ofendogenous H2O2 level which couples to APX protein synthesis in a translation stage in cotyledons.