The sequence of changes within the photosynthetic apparatus of Populus nigra leaves, fumigated with ozone in open-top chambers, was investigated. Besides two chambers with charcoal-filtered air (CF), two with non filtered air (NF) and two with non filtered air +50 nl l(-1) ozone (NF+) were used. The first effect to be found on photosynthesis was a significant reduction in carboxylation efficiency (CE) in 10 day old NF- and NF+-leaves compared to CF-leaves. This effect was enhanced with the ageing of the leaves. On the other band photochemical capacity measured by dark-adapted chlorophyll fluorescence (F(v)/F(m)) was not reduced in NF-leaves compared to CF-leaves during the whole growing season. In NF+-leaves F(v)/F(m) showed a decrease in comparison to CF-leaves from 42 days onwards, when CE was already reduced by about 60% in the same leaves. In 34 and 75 day old leaves there was an increase in the ratio of the xanthophylls violaxanthin + antheraxanthin + zeaxanthin to chlorophyll a+b in leaves that w