Free amino acids composition and content in Betula sp. male generative organs were studied. 23 amino acids were detected. They were presented in all trees. The amino acid composition in birch catkins was found to change during their development. Citrulline was the main free amino acid transported in the xylem to stem, leaves and catkins. Citrulline plays an important role in nitrogen translocation. Proline prevails in male catkins after dormancy in spring. The newly formed male catkins accumulate arginine during the summer.