Rationale: In diabetic patients, heart failure with predominant left ventricular (LV) diastolic dysfunction is a common complication for which there is no effective treatment. Oxidation of the nitric oxide synthase (NOS) co-factor tetrahydrobiopterin (BH4) and dysfunctional NOS activity have been implicated in the pathogenesis of the diabetic vascular and cardiomyopathic phenotype.Objective: Using mice models and human myocardial samples, we evaluated whether and by which mechanism increasing myocardial BH4 availability prevented or reversed LV dysfunction induced by diabetes. Methods and Results: In contrast to the vascular endothelium, BH4 levels, superoxide production and NOS activity (by liquid chromatography) did not differ in the LV myocardium of diabetic mice or in atrial tissue from diabetic patients. Nevertheless, the impairment in both cardiomyocyte relaxation and [Ca2+]i decay and in vivo LV function (echocardiography and tissue Doppler) that developed in wild type mice (WT) 12 weeks post-DM induction (streptozotocin, 42-45mg/kg) was prevented in mice with elevated myocardial BH4 content secondary to overexpression of GTP-cyclohydrolase 1 (mGCH1-Tg) and reversed in WT mice receiving oral BH4 supplementation from the 12th to the 18th week after DM induction. The protective effect of BH4 was abolished by CRISPR/Cas9-mediated knockout of neuronal NOS (nNOS) in mGCH1-Tg. In HEK cells, S-nitrosoglutathione led to a PKG-dependent increase in plasmalemmal density of the insulin-independent glucose transporter, GLUT-1. In cardiomyocytes, mGCH1 overexpression induced a NO/sGC/PKG-dependent increase in glucose uptake via GLUT-1, which was instrumental in preserving mitochondrial creatine kinase activity, oxygen consumption rate, LV energetics (by 31P MRS) and myocardial function. Conclusions: We uncovered a novel mechanism whereby myocardial BH4 prevents and reverses LV diastolic and systolic dysfunction associated with diabetes via a nNOS-mediated increase in non-insulin dependent myocardial glucose uptake and utilization. These findings highlight the potential of GCH1/BH4-based therapeutics in human diabetic cardiomyopathy.