Abstract—One key mechanism for endothelial dysfunction is endothelial NO synthase (eNOS) uncoupling, whereby eNOS generates O₂^•- rather than NO because of deficient eNOS cofactor tetrahydrobiopterin (BH4). This study was designed to examine the effect of BH4 deficiency on cardiac morphology and function, as well as the impact of metallothionein (MT) on BH4 deficiency–induced abnormalities, if any. Friend virus B (FVB) and cardiac-specific MT transgenic mice were exposed to 2,4-diamino-6-hydroxy-pyrimidine (DAHP; 10 mmol/L, 3 weeks), an inhibitor of the BH4 synthetic enzyme GTP cyclohydrolase I. DAHP reduced plasma BH4 levels by 85% and elevated blood pressure in both FVB and MT mice. Echocardiography found decreased fractional shortening and increased end-systolic diameter in DAHP-treated FVB mice. Cardiomyocytes from DAHP-treated FVB mice displayed enhanced O₂^•- production, contractile and intracellular Ca²⁺ defects including depressed peak shortening and maximal velocity of shortening/relengthening, prolonged duration of relengthening, reduced intracellular Ca²⁺ rise, and clearance. DAHP triggered mitochondrial swelling/myocardial filament aberrations and mitochondrial O₂^•- accumulation, assessed by transmission electron microscopy and MitoSOX Red fluorescence, respectively. DAHP also promoted the N^G-nitro-L-arginine methyl ester–inhibitable O₂^•- production and eNOS phosphorylation at Thr497. Although MT had little effect on cardiac mechanics and ultrastructure, it attenuated DAHP-induced defects in cardiac function, morphology, O₂^•- production, and eNOS phosphorylation (Thr497). The DAHP-induced cardiomyocyte mechanical responses were alleviated by in vitro BH4 treatment. DAHP inhibited mitochondrial biogenesis, mitochondrial uncoupling protein 2, and chaperone heat shock protein 90, and all but uncoupling protein 2 were rescued by MT. Our data suggest a role for BH4 deficiency in cardiac dysfunction and the therapeutic potential of antioxidants against eNOS uncoupling in the heart.