The present study was carried out to compare the antihyperglycemic effects of melatonin and extract of Cleome droserifolia separately or in combination on the antioxidant defense system of diabetic rats. Melatonin (10 mg kg-1) and C. droserifolia (0.045 g kg-1day-1) were given i.p. after induction of diabetes with alloxan (110 mg kg-1). Administration of alloxan to rats resulted hyperglycemia, hypoinsulinemia, reduced thyroid hormone levels and decreased body weight. These changes were accompanied with significant decrease in serum total protein, nitric oxide (NO), total antioxidant capacity (TAC), liver glutathione (GSH) level and catalase activity (CAT) of diabetic rats. Serum protein level of the melatonin or extract groups did not differ significantly during the period of the study as compared with the diabetic group, while it returned to the normal level in the case of combined-exposure. Melatonin increased the levels and activities of antioxidant markers including total protein, GSH, catalase and NO as compared with diabetic group. Melatonin and crude extract of C. droserifolia caused a significant decrease in liver tissue malondialdehyde (MDA) levels. Furthermore, treatment of diabetic rats with crude extract or its combination with melatonin had more potent effect on liver GSH level, insulin, T3, T4 and T3/T4 ratio than treatment with melatonin only. C. droserifolia was found to be less effective on CAT levels in liver than melatonin. TAC had positive correlation with GSH and CAT, while it had negative correlation with MDA. The present results confirmed that diabetes increased oxidative stress in liver and indicated the role of melatonin in combating the oxidative stress via its free radical-scavenging and antioxidant properties. Also, crude extract prevented any diabetic complications by reducing oxidative stress and protected rats from oxidative damage. Considering the much lower concentration of melatonin compared with extract, melatonin seems to be more potent antioxidant. Concomitant exposure to melatonin potentiates crude extract as antihyperglycemic effect on diabetic rats.