Metformin improves cardiovascular outcomes in type 2 diabetes, but its exact mechanisms of action remain controversial. We used hyperpolarized [1-13C]pyruvate magnetic resonance spectroscopy to determine the effects of metformin treatment on heart and liver pyruvate metabolism in rats in vivo. Both oral treatment for 4 weeks and a single intravenous metformin infusion significantly increased the cardiac [1-13C]lactate:[1-13C]pyruvate ratio but had no effect on the [1-13C]bicarbonate + 13CO2:[1-13C]pyruvate ratio, an index of pyruvate dehydrogenase flux. These changes were paralleled by a significant increase in the heart and liver cytosolic redox state, estimated from the [lactate]:[pyruvate] ratio but not the whole-cell [NAD+]/[NADH] ratio. Hyperpolarized MRI localized the increase in cardiac lactate to the left ventricular myocardium, implying a direct myocardial effect, though metformin had no effect on systolic or diastolic cardiac function. These findings demonstrate the ability of hyperpolarized pyruvate magnetic resonance spectroscopy to detect metformin-induced changes in cytosolic redox biology, suggest that metformin has a previously unrecognized effect on cardiac redox state, and help to refine the design of impending hyperpolarized magnetic resonance studies in humans.