Tropospheric ozone and high light intensities are two stress factors that often occur simultaneously under natural conditions. Ozone is well known to form oxygen radicals in the apoplastic water and long lasting photoinhibition can cause photooxidative damage also by formation of several species of oxygen radicals. We were interested whether moderate levels of ozone would be able to modulate the response of leaves to photoinhibitory conditions naturally occurring around noon on a bright day. Cuttings of Populus sp. were cultivated in two separate greenhouse-compartments adapted as fumigation chambers. In the two compartments plants were grown in ambient air containing about 20 nmol mol(-1) ozone and in elevated ozone concentrations supplied for 8 h per day. During the midday of bright days Fv/Fm decreased by the same amount in all leaves, indicating photoinhibition. At the same time Fo increased in control leaves more than in ozone-exposed leaves indicating a higher amount of heat-deactivating PSII centres in the latter. This was confirmed by a higher epoxidation state in ozone-exposed leaves during midday of a bright day. The contents of chlorophyll a and chlorophyll b were significantly decreased in ozone-exposed leaves. In older leaves the ratios chlorophyll a : b, and xanthophylls: chlorophyll b were increased indicating an adaptation to higher light stress. From this we conclude that by increasing the amount of heat-deactivating centres ozone seems to protect PSII from photoinhibition.