Clinical research has confirmed the efficacy of several photo-chemicals in modulating oxidative stress associated with diabetes mellitus. Here we investigate the effect of tetrahydrocurcumin (THC), an active metabolite of curcumin, on antioxidant status in streptozotocin-nicotinamide-induced diabetes in rats. A single dose of streptozotocin (65 mg kg(-1) bwt) resulted in decreased insulin, hyperglycemia, increased lipid peroxidation (thiobarbituric reactive substances, lipid hydroperoxides), and decreased antioxidant levels (vitamin C, vitamin E, reduced glutathione and ceruloplasmin). The oral administration of THC (80 mg kg(-1) bwt) for 45 days to diabetic rats significantly increased plasma insulin and plasma antioxidants and significantly decreased lipid peroxidation. The positive effects of THC were better that those achieved with curcumin. The results of the study indicate that in addition to its antidiabetic effect in type 2 diabetic rats, THC has an antioxidant effect.