The effects of 96-hour exposure to 0.25, 0.125 and 0.06 portions of the 96-hour LC₅₀ of Cu, Zn singly and Cu + Zn mixture on biological (survival, gill ventilation frequency, gill-somatic index) parameters of rainbow trout were studied depending on the stage of development of the fish. Hatching embryos had the greatest sensitivity to the affect of metals singly and in admixture (the highest percentage of mortality). Marked changes were recorded in gill ventilation frequency during exposure to these metals singly and in admixture, which corresponded to changes in their gill-somatic indices. Changes to certain parameters studied (mortality of embryos, especially hatching embryos, gill-somatic index of juveniles) indicated enhanced toxicity of the metal mixture to rainbow trout in ontogenesis.