Changes in activity of one of the most important enzymes of nitrogen compound metabolism - glutamate dehydrogenase (GDH, EC1.4.1.2-4) under the conditions of variable concentrations of Cd²⁺, Zn²⁺, Pb²⁺ were investigated. Dry matter increase and glutathione (GSH) levels varying under these conditions were also analyzed. The obtained results show a close relationship between the concentration of the investigated metal ions, especially Cd²⁺, and the metabolism of primary amino acids. The increase in heavy metal concentration resulted in a many-fold GDH activity increase, significant decrease in seedling mass, and lowering of the GSH level. The increase in GSH activity may attest to the important role of that enzyme in catabolic processes intensified under these conditions, or to the increasing importance of the reductive amination of 2-oxoglutarate in the states of physiological stress caused by the presence of heavy metals. The observed relationship concerns the plant's photosynthesizing organs rather than its root system.