Administration of streptozotocin ( STZ) was used to induce diabetes, as the mechanism involved is believed associated with generation of free radicals. Supplementation with antioxidant molecules such as melatonin may serve as a protection against diabetes. The aim of this study was to determine whether the STZ-induced effects on plasma thiobarbituric acid-reactive substances ( TBARS, a marker of lipid peroxidation) and total sialic acid levels could be blocked by melatonin. STZ significantly increased the plasma levels of sialic acid and TBARS. Treatment with melatonin markedly reduced the STZ-induced effects on plasma sialic acid and TBARS and was associated with restoration of hyperglycemia to control blood glucose levels. These data suggest that melatonin protects against oxidative damage, and daily supplementation with melatonin may be beneficial for diabetics.