Metabolism, monitored via in situ catalytic enzyme histochemistry and fine structure, was studied in the myocardium of chronic diabetic male Wistar rats administered L-arginine (12.8 mg/100 g/day) for 24 weeks. Diabetes was induced with a single i.v. injection of 55 mg/kg streptozotocin. After 6 months, the tissue of the left ventricle was processed for electron microscope examination and transmural tissue blocks were frozen for enzyme histochemistry. In diabetic myocardium, heterogeneous ischemia-like subcellular alterations of cardiomyocytes and capillaries were observed, together with interstitial fibrosis. This structural remodeling was accompanied by significantly decreased activity of endothelial nitric oxide synthase (NOS) and heterogeneously decreased activities of glycogen phosphorylase (GlPh), hydroxybutyrate dehydrogenase (HBDH) and adenosine triphophatases (ATPases) throughout the myocardium. In arginine-treated diabetic rats, there was evidence of protected structural integrity of endothelial cells and attenuated structural disturbances of cardiomyocytes. This was associated with the markedly preserved histochemical activities of all detected enzymes in comparison with nontreated diabetic rats (NOS 98.7 +/- 10.5% vs. 35.4 +/- 4.1%; ATPases 82.7 +/- 9.1% vs. 69.3 +/- 5.2%; GlPh 65.2 +/- 8.3% vs. 45.5 +/- 3.8%; HBDH 68.9 +/- 8.5% vs. 44.1 +/- 6.7% of control values). The results indicate that long-term supplementation of L-arginine may account for the reduction of diabetes-induced myocardial structural remodeling.