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The effects of increased endogenous cortisol levels were compared with those of sublethal copper exposurein the freshwater common carp, Cyprinus carpio. Fish were exposed to either increased levels of endogenous cortisol (200 ng/ml) or sublethal copper (1.9 μM) alone or were pretreated by elevating plasma cortisol levels prior to copper exposure to assess whether interactions between both treatments occurred. Effects induced by increased cortisol levels included increased Na⁺/K⁺‐adenosine triphosphate (ATPase) activity and increased plasma Na⁺ and plasma osmolarity, while copper exposure induced anaerobic metabolism, gill damage, decreasing Na⁺/K⁺‐ATPase activity, decreasing plasma ion levels, and blood thickening. Pretreatment of copperexposed fish with cortisol partially protected these fish by reducing the copper‐induced decrease in Na⁺/K⁺‐ATPase activity. Overall, the results obtained in …