OBJECTIVE To identify the role of metformin in cardiac hypertrophy and investigate the possible mechanism underlying this effect. METHODS Wild type and AMPKα2 knockout (AMPKα2⁻/⁻) littermates were subjected to left ventricular pressure overload caused by transverse aortic constriction. After administration of metformin (200 mg·kg⁻¹·d⁻¹) for 6 weeks, the degree of cardiac hypertrophy was evaluated using echocardiography and anatomic and histological methods. The antihypertrophic mechanism of metformin was analyzed using Western blotting. RESULTS Metformin significantly attenuated cardiac hypertrophy induced by pressure overload in wild type mice, but the antihypertrophic actions of metformin were ablated in AMPKα2⁻/⁻ mice. Furthermore, metformin suppressed the phosphorylation of Akt/protein kinase B (AKT) and mammalian target of rapamycin (mTOR) in response to pressure overload in wild type mice, but not in AMPKα2⁻/⁻ mice. CONCLUSIONS Long-term administration of metformin may attenuate cardiac hypertrophy induced by pressure overload in nondiabetic mice, and this attenuation is highly dependent on AMPK activation. These findings may provide a potential therapy for patients at risk of developing pathological cardiac hypertrophy.