Abstract <p>Pollution of the aquatic environment by metals must be considered taking into account their probable combinations and emerging chronic effects. The spectrum of effects of copper and its combined action with lanthanum is determined for the first time in the amphipod <i>Hyalella azteca</i> Saussure, 1858 in a chronic experiment (40 days). The effect of solutions containing equimolar calculated concentrations of Cu²⁺ ions and combinations of Cu²⁺ and La³⁺ (1 : 1) is modeled in two variants: 0.16 and 0.8 µmol/L per metal or their sum. Solutions with a higher concentration naturally have a more pronounced effect on amphipods. The effects of the metal mixture exceed their individual effect (0.8 µmol/L). In solutions with a combination of Cu²⁺ and La³⁺, the death of individuals is 2.5 times greater, and the linear dimensions and mass of individuals are 1.1 and 1.4 times lower compared with similar indicators for the Cu²⁺ variant. In combined solutions, the leveling of sexual dimorphism according to these signs is recorded. The trophic activity of amphipods tends to increase in variants with a mixture of metals compared with the action of copper and control. Thus, an increase in the action of copper in the presence of lanthanum is shown. Consequently, the risks associated with REEs entering the environment require revision.</p>