A decoction of leaves and unripe fruits prepared from C. aronia syn. azarolus (L) (Rosaceae), the indigenous Israeli hawthorn, is used to treat diabetes in Arab traditional medicine. There is a growing body of evidence reporting the presence of systemic oxidative stress in insulin-dependent and non-insulin dependent diabetes. We have already reported that this decoction has antioxidant properties in vitro. In this report, we extended these in vitro observations to study its antioxidant action in the streptozotocin (STZ)-induced diabetic rat in which we measured its effect on plasma and tissue indices of oxidative stress, as well as blood glucose levels. In an experiment lasting 24 days, 30 diabetic and 30 control rats were each subdivided into three separate groups. Group 1 rats drank tap water, group 2 rats drank the decoction containing 50 mg grounded dried plant/mL and group 3 rats drank the decoction containing 350 mg grounded dried plant/mL. Upon completion of the treatment period, the rats were anesthetized and blood glucose was measured using a drop of blood obtained by tail vein puncture. The abdomen was opened and aortic blood was withdrawn for measurement of the plasma levels of lipid peroxides, glutathione, cholesterol and triglycerides and the livers were harvested for determination of the extent of lipid peroxidation. The results of these experiments show that a decoction prepared from the leaves and unripe fruits of C. aronia normalizes plasma lipid peroxide levels and lowers blood glucose levels in diabetic rats. These findings indicate that the therapeutic benefit of the decoction could be due to these two actions, thereby providing a basis for its use in treating diabetes in traditional Arab medicine.