Progression of some renal diseases is characterized by generation of reactive oxygen metabolites that are also involved in the pathophysiology of obstructive nephropathy. Catalase activity and lipid peroxidation were investigated in rats with unilaterally (UUL) and bilaterally ligated ureters (BUL). Forty-eight hours after ligation, the animals were sacrificed, and enzyme activity as well as the malondialdehyde (MDA) concentration were measured in the plasma, kidneys and livers. The activity of catalase was significantly reduced in the plasma of the BUL rats and in the kidneys of both investigated groups. In the liver, catalase activity was decreased only in the BUL group. The MDA concentration in the plasma and kidneys of the BUL rats was significantly increased while in the liver it remained unchanged. These results suggest that lipid peroxidation in the induced uremic state could be responsible for catalase inactivation.