Diabetic cardiomyopathy is responsible for substantial morbidity and mortality in the diabetic population. Increased oxidative stress has been associated with the pathogenesis of chronic diabetic complications including cardiomyopathy. Multiple biochemical mechanisms have been proposed to increase oxidative stress in diabetes. The present study was aimed at elucidating the role of a potent oxidative and cellular stress-responsive system, the heme oxygenase (HO) system, in the heart in diabetes. Streptozotocin-induced diabetic rats were treated with a potent inhibitor of HO system, tin protoporphyrin IX (SnPPIX, 50 micromol/kg/d), and were compared with untreated diabetic and non-diabetic animals. All treatments began at the onset of diabetes, 48 h after injection of streptozotocin along with the confirmation of hyperglycemia. Animals were euthanized after 1 week and 1 month of treatment, and heart tissues were harvested. Frozen tissues were subjected to HO-1 and HO-2 mRNA expression by real-time RT-PCR and HO activity determination. Paraffin-embedded tissue sections were used for immunohistochemical analysis of HO-1 and HO-2. 8-Hydroxy-2'-deoxyguanosine (8-OHdG) stain, a sensitive and specific marker of DNA damage, was preformed to assess damage induced by oxidative stress. In addition, tissue sections were subjected to histochemical analysis for iron. We further examined non-diabetic animals treated with a direct HO agonist, hemin (50 mg/kg/d). A possible relationship between the HO and the nitric oxide (NO) pathways was also considered by studying the mRNA levels of endothelial nitric oxide synthase (NOS) and inducible NOS, and by measuring the amount of NOS products. Our results demonstrate no significant alterations of the HO system following 1 week of diabetes. However, 1 month of diabetes caused increased oxidative stress as demonstrated by higher levels of 8-OHdG-positive cardiomyocytes (80% positive as compared to 11.25% in controls), in association with increased HO isozyme mRNA (2.7-fold increase as compared to controls) and protein expression, and augmented HO activity (759.3 as compared to 312.3 pmol BR/h/mg protein in controls). Diabetic rats further demonstrated increased number of cardiomyocytes with stainable iron. SnPPIX treatment resulted in reduced number of 8-OHdG-positive cardiomyocytes (19.5% as compared to 80% in diabetics) in parallel with reduced HO activity (569.7 as compared to 759.3 pmol BR/h/mg protein in diabetics). Non-diabetic rats treated with HO-agonist hemin exhibited abnormalities similar to diabetic rats. Our results provide the first direct demonstration that diabetes-induced oxidative stress in the heart is, in part, due to upregulated HO expression and activity. These results provide evidence of pro-oxidant activity of HO in the heart in diabetes, which could be mediated by increased redox-active iron.