Copper deficiency in the rat results in a high decrease of erythrocyte superoxide dismutase activity (by 70%), an increase of glutathione peroxidase activity (by 17%) and glucose 6-phosphate dehydrogenase activity (by 40%) and no change in catalase activity. 2. Ascorbate (30 nM) and copper (10 and 50 nmol/mg protein) enhance about 2-fold the lipid peroxidation of erythrocyte membranes from copper-deficient rats. 3. The osmotic stability of copper-deficient rat erythrocytes is higher compared with that of the controls.