Alloxan-induced diabetic rats were treated with insulin (i.p.) or with Capparis decidua powder as a hypoglycaemic agent mixed with diet. The effect was assessed on lipid peroxidation (LPO) and the antioxidant defense system in rat tissues. The increased levels of blood glucose in diabetes produce superoxide anions and hydroxyl radicals in the presence of transition metal ions which cause oxidative damage to cell membranes. The heart tissue showed an increased lipid peroxidation (LPO) in diabetic rats while no significant change was observed in the liver and kidney. The treatment with C. decidua lowered LPO in these tissues even more effectively than insulin-treated rats. The superoxide dismutase (SOD) activity increased in the heart and kidneys in the diabetic group of rats probably to increase dismutation of superoxide anions. However, treatment with C. decidua decreased SOD activity in the liver and kidney and was comparable to control rats. Catalase (CAT) activity was not significantly affected in any of the tissues in diabetic and insulin-treated animals, however, CAT activity markedly increased in tissues with C. decidua treatment. Total and Se-dependent glutathione peroxidase (GSH-Px) in the heart was markedly lowered in diabetic rats which recovered with insulin as well as with C. decidua treatment. The increase in GSH-Px and CAT activity with C. decidua treatment may lower H2O2 toxicity and reduce oxidative stress in diabetes. However, glutathione (GSH) content in the heart and kidney and glutathione reductase (GSH-R) activity in all the tissues studied increased in diabetic rats while treatment with insulin lowered GSH content and GSH-R activity in these tissues. The treatment with C. decidua also decreased GSH-R activity in the kidney and heart which resulted in the decrease in GSH content in these tissues. The changes such as the increase in kidney and heart SOD may be an adaptive response in order to neutralize superoxide anions. The increase in GSH content and GSH-R activity in the tissue are in response to neutralize superoxide anions and to counteract oxidative stress in diabetes. Glutathione S-transferase (GST) was not significantly affected in diabetic rat tissue, however, heart GST increased with antidiabetic treatments. The increase in glucose-6-phosphate dehydrogenase (G6PDH) in the kidney and heart of diabetic rats subsequently decreased with C. deciduas treatment. The increase in G6PDH in tissues may increase NADPH generation required for GSH-R activity and GSH production. It is suggested that these changes initially counteract the oxidative stress in diabetes, however, a gradual decrease in the antioxidative process may be one of the factors which results in chronic diabetes. The data indicate that C. decidua may have potential use as an antidiabetic agent and in lowering oxidative stress in diabetes. Copyright 1997 The Italian Pharmacological Society.