The introduction of rapidly growing trees in so-called short-rotation forestry requires knowledge about their water use in order to select suitable land for plantations and to manage the stands in a proper way. In Sweden, different clones of Salix are usually used in short-rotation forestry today. The diameter of trees in such stands is much smaller compared with trees in traditional forestry. This requires new methods to be developed in order to study water relations at the tree level. The aim of this study was to modify, apply and test the stem tissue heat balance method for long-term sap flow measurements on small size Salix trees. Sap flows measured on two trees were compared to stand evaporation determined by the Bowen ratio method and to other climatic variables. Diurnal courses of sap flow and evaporation corresponded well to each other but with a pronounced lag of sap flow behind evaporation in the morning. Also, the daily integrals of sap flow, evaporation and global radiation showed good correlation over a two month period. Maximum sap how rate of 30 mm diameter trees was about 0.2 kg h(-1) or 2 kg day(-1). The size of the pool of easily available water in the trees was estimated to be 0.2 kg. This value constitutes about 1/4 of the mean daily transpiration, a value similar to those found for much larger trees. The conclusion of this study was that the stem tissue heat balance method with externally placed heater and internally sensed temperature is an appropriate method for measurement of sap flow and transpiration of individual small size Salix trees.