The unmixed silver birch (Betula pendula Roth) and downy birch (Betula pubescens Ehrh.) stands were chosen in the Middle Lithuania for the purposes of this research. Chemical composition of fresh birch tree sap was determined right after collection of samples. The first sampling was done at the beginning of sap exudation and the second one at the stage of the most intensive sap exudation. Concentrations of sugars in sap of Betula pendula and Betula pubescens trees growing on temporarily overmoistured soil (Lcp) were higher at the beginning of sap exudation. Sap of Betula pubescens was sweeter than Betula pendula (2.4 % sugars). Level of ascorbic acid (7.0 mg 100 g^-1) in Betula pendula sap did not change during the sap exudation period, while in Betula pubescens sap it increased by 10.0 % at the stage of the most intensive sap exudation. During the same stage the total concentration of sugars in sap of Betula pendula trees growing on the light soil of normal humidity (Ncl) rose by 10 %. Higher concentrations of sugars were determined in sap of birch trees growing on more humid soils: 2.0 % at the beginning and 1.9 % at the later stage of sap exudation period. The highest content of soluble solids was determined in sap collected from the Betula pendula trees growing on temporarily overmoistured soil (Lcp). Soil type and sap sampling time had no effect on the content of ascorbic acid in Betula pendula sap. The mineral composition of sap improved significantly at the stage of the most intensive sap exudation. The sap of Betula pendula and Betula pubescens contained 80.4 and 60.0 mg kg^-1 potassium and 74.3 and 57.7 mg kg^-1 calcium, respectively. These two elements constituted the major part of the total amount of measured macro- and microelements. Betula pendula sap contains larger amounts of mineral elements than the sap of Betula pubescens irrespective of soil type and sap sampling time.

Keywords: Betula pendula, Betula pubescens, birch sap chemical compounds, mineral composition.