A study is made of the level of endogenous indole-acetic (IAA) and abscisic acids (ABA) in the shoot apex of the tall and dwarf forms of Siberian crabapple in the natural environment and in the case of introduction. It is shown that a dwarfing in plants is caused by a decline in IAA content, and by the associated changes of the IAA/ABA ratio. This ratio has always been above and below unity for tall and dwarf trees, respectively. The ratio of hormones and their contents in the studied forms did not change when introduced, while increasing the rate of growth of the shoots. It is concluded that the IAA level is not determined by its acropetal transport but depends on the synthesis in the above-ground part of a plant. In this context, we discuss the contribution of phytohormones to formation of the dwarf status of Siberian crabapple.