Activity of chloroplast-localized DNA endonuclease was observed in detached tobacco leaves that had been treated with paraquat and light. The DNA endonuclease was able to cleave the chloroplast, plasmid, and single-stranded DNA, as estimated on an agarose gel. Activity was sensitive to two endonuclease inhibitors: aurintricarboxylic acid and ZnSO$_4$. The time course for activity showed a peak 4 h after the stress treatment. These results suggest that this enzyme plays a specific physiological role during oxidative stress. Probable roles for this enzyme are also discussed.