Following myocardial infarction (MI), the heart undergoes a pathological process known as remodeling, which in many instances results in cardiac dysfunction and ultimately heart failure and death. Transforming growth factor-beta (TGF-beta) is a key mediator in the pathogenesis of cardiac remodeling following MI. We thus aimed to inhibit TGF-beta signaling using a novel orally active TGF-beta type I receptor activin receptor-like kinase 5 (ALK5) inhibitor (GW788388) to attenuate left ventricular remodeling and cardiac dysfunction in a rat model of MI. Sprague-Dawley rats underwent left anterior descending coronary artery ligation to induce experimental MI and then were randomized to receive GW788388 at a dosage of 50 mg.kg(-1).day(-1) or vehicle 1 wk after surgery. After 4 wk of treatment, echocardiography was performed before the rats were euthanized. Animals that received left anterior descending coronary artery ligation demonstrated systolic dysfunction, Smad2 activation, myofibroblasts accumulation, collagen deposition, and myocyte hypertrophy (all P < 0.05). Treatment with GW788388 significantly attenuated systolic dysfunction in the MI animals, together with the attenuation of the activated (phosphorylated) Smad2 (P < 0.01), alpha-smooth muscle actin (P < 0.001), and collagen I (P < 0.05) in the noninfarct zone of MI rats. Cardiomyocyte hypertrophy in MI hearts was also attenuated by ALK5 inhibition (P < 0.05). In brief, treatment with a novel TGF-beta type I receptor inhibitor, GW788388, significantly reduced TGF-beta activity, leading to the attenuation of systolic dysfunction and left ventricular remodeling in an experimental rat model of MI.