We studied the response of each ascorbate peroxidase (APX) isoenzyme in spinach leaves under stress conditions imposed by high light intensity, drought, salinity, and applications of methyl viologen and abscisic acid. The steady-state transcript level of cytosolic APX remarkably increased in response to high-light stress and methyl viologen treatment, but not in response to the other stress treatments. The transcript levels of the chloroplastic (stromal and thylakoid-bound) and microbody-bound APX isoenzymes were not changed in response to any of the stress treatments. To explore the responses of the APX isoenzymes to photooxidative stress, the levels of transcript and protein and activities of each isoenzyme were studied during high-light stress and following its recovery. The cytosolic APX activity increased in parallel with transcript abundance during high-light stress, while the protein level was not altered. The other isoenzymes showed no significant changes in transcript and protein levels and activities, except for the gradual decrease in chloroplastic isoenzyme activities.