Summary Peroxidase (EC 1.11.1.7) activity in jack pine (Pinus banksiana) and white birch (Betula papyri fera) seedlings increased greatly during aging of the foliar tissues. Fumigation of seedlings with SO, caused a marked increase in enzyme activity and sulphur content of both species; these responses were, however, more pronounced in white birch than in jack pine. The stimulation of peroxidase in SO, -fumigated jack pine seedlings appeared to be due to increased production of the peroxidase isoenzymes rather than their activation. Increase in both peroxidase activity and sulphur content in the fumigated needle tissues appeared to be transitory in nature as shown by their decline during recovery in an SO2 free atmosphere. A possible role of peroxidase in SO2-initiated free radical process is discussed.