This study investigated the influence of protein restriction on lipid peroxide content (thiobarbituric acid-reactive substances) and the intracellular antioxidant defence system in the small intestine and the liver. Weanling male Sprague-Dawley rats were divided into two groups: a low-protein group fed ad lib. a diet containing 6% casein for 4 wk, and a control group fed a diet containing 22% casein but restricted to the same dietary intake as that of the low-protein group. Compared with pair-fed controls, the protein-deficient rats exhibited a decrease in glutathione content in the small intestine and the liver. Thiobarbituric acid-reactive substances were increased in the intestine only. Data on the enzymatic antioxidant defence system in the protein-deficient animals showed an increase in catalase activity in the intestine but a decrease of this activity in the liver. The activity of selenium-dependent glutathione peroxidase was decreased in the liver and remained unchanged in the intestine. Superoxide dismutase was not modified by protein deficiency in either tissue. In control rats, enzyme activities were 6-43 times higher in the liver than in the intestine. The deleterious effects of protein deficiency appeared more marked in the intestine.