The objective of the study conducted in the period of 2010-2012 on one-, two- and three-year old shoots of three varieties of Salix viminalis: Bjor, Jorr and Tora, grown from 14-, 15- and 16-year old rootstocks, was their comparison with regard to the processes of assimilation and transpiration, and of the parameters affecting those processes, i.e. stomatal conductance, sub-stomatal concentration of CO₂, indices WUE and WUEI, and in terms of their yields. Measurements of gas exchange were performed with the use of the gas analyser type TPS-2 with PLC-4 camera (PP System, USA), operating on the open system. Based on the results obtained it was concluded that one-year shoots of variety Jorr were characterised by the most favourable parameters of gas exchange, while the poorest were noted for the shoots of variety Tora. The high overall activity of the processes of gas exchange of variety Jorr allowed it to achieve the highest yield of shoot biomass, irrespective of the harvest cycle. In spite of the advanced age of the plantation (14^th -16th year of cultivation) the varieties studied were characterised by high biomass productivity. Probably, due to the large increase of wood matter of three-year old shoots of variety Bjor, accounting for ca. 58% of the entire three-year growth, significantly greater than in the case of Jorr and Tora (ca. 50%), for that variety a 4-year harvest cycle would be the most favourable.