The purpose of this study was to assess the influence of SO2 on photosynthetic apparatus and the level of total nonstructural carbohydrates (TNC) in developing and mature leaves of poplar (Populus deltoides). Photosynthetic apparatus was evaluated on the basis of fluorescence parameters (Fv/Fm, OPSII, qP and Rfd) and photosynthetic pigments (chlorophylls and carotenoids). Cuttings of poplar were exposed to 0.25 ppm of SO2 at 25 degrees C and 200-300 mmol m-2 s-1 PAR for 6 hours daily during 5 days in a fumigation chamber. The fumigation did not produce any significant differences in fluorescence parameters in neither developing nor mature leaves. In some mature leaves the concentration of pigments increased under the influence of SO2. Soluble carbohydrates decreased significantly both in developing and mature leaves and this was accompanied with an increase in starch accumulation. We suggest that Populus deltoides is a species tolerant to sulphur dioxide and the used SO2 dosage did not significantly impair the light reactions of photosynthesis, but it disturbed the accumulation of starch and the utilization of soluble carbohydrates in plants exposed to SO2.