Physiological and biochemical perturbations in the liver ofCarassius auratus were investigated in vivo following 40 days ofexposure to ytterbium solutions of di4erent concentration.Glutamate8pyruvate transaminase (GPT) activity in gold5shliver was stimulated at 0.05mg/L Yb3+ and inhibited at higherYb3+ concentrations. Activity of the antioxidant enzymesuperoxide dismutase (SOD) was stimulated at Yb3+ higher than 0.05mg/L, and catalase (CAT) activity was strongly inhibitedafter 40 days of exposure. Detoxifying enzymes glutathioneS-transferase (GST) and glutathione peroxidase (GSH-Px) werestimulated at 0.05mg/L and inhibited at 0.1mg/L after 40 daysof exposure. Among the parameters determined, CATin gold5shliver was most sensitive to Yb3+, indicating that CATmightbe considered a potential tool in the biomonitoring of exposureto Yb3+ in an aquatic ecosystem.