The objective of the work was to determine the content of magnesium in plants of Trifolium pratense L. which grew in an environment contaminated with aluminium or copper. The growth and development of the plants was carried out in hydroponics, with strictly controlled composition and pH of the growth medium. Copper was added to the medium (as an additional application) in a solution of CuSO₄·5H₂O, and aluminium in the form of a solution of AlCl₃, at concentrations of Al⁺³ and Cu⁺² of 20 and 100 mg dm^-3. The concentration of magnesium, aluminium and cooper was determined by means of a ICP sequential emission spectrometer D-820 Hilger Analytical (UK) in extracts from the roots and aboveground parts. Decreasing pH of the medium did not cause any statistically significant changes in the concentration of the metals studied. There was a significant increase in the concentration of aluminium and copper in roots and aboveground parts of Trifolium pratense L. and lower magnesium concentration for plants that were grown in an environment contaminated with Al⁺³/ Cu⁺² at a concentration of 100 mg dm^-3. In the lower concentrations of both stressors (20 mg dm^-3) a decrease was noted in the level of magnesium in the whole plant, and a statistically significant increase in the concentration of aluminium and copper in the roots of Trifolium pretense L. In the aboveground parts of the plants that grew in medium supplemented with 20 mg dm^-3 of Al⁺³/Cu⁺², the increase of the concentration of these metals was not statistically significant.