Sulfite at concentrations from 0.5 to 5.0 mM was supplied to illuminated, detached poplar (Populus deltoides Bartr. ex Marsh) leaves via the transpiration stream. Concentrations of chlorophyll a and b, carotenoids, glucose, fructose, sucrose and starch, and extractable specific activities of sucrose synthase (SuSy), acid invertase (AI) and neutral invertase (NI) were measured. In sink and source leaves chlorophylls and carotenoids appeared to be increased by sulfite at concentrations ranging from 0.5 to 5.0 mM. In source leaves the application of 5.0 mM and ≥ 2.5 mM sulfite led to a decrease in total nonstructural soluble sugars (sTNC) and starch, respectively. The specific activity of SuSy was enhanced by ≥ 0.5 mM sulfite. There was only a slight decline in the activity of Al and N1.In sink leaves sTNC decreased and starch increased under the influence of sulfite at concentrations ≤ 1.O mM and ≥ 0.5 mM, respectively. The effect of sulfite on the activity of At, NI and SuSy was negligible. However, in both development groups of leaves alterations in sucrose, glucose and fructose contents were observed. On the basis cf the obtained results we postulate that sulfite changes the sucrolytic activities primarily in source leaves and only marginally in sink leaves.